Re: [PATCH v4 1/5] docs: IOMMU user API
On Tue, 14 Jul 2020 13:04:12 -0600 Alex Williamson wrote: > > > The > > > mangling of the user provided argsz above makes me cringe a > > > little too for that reason, once we start modifying the user > > > values in the core it could get messy for the vendor drivers. > > > > > We do have vendor specific union in bind_gpasid UAPI. Could you > > elaborate your concern? > > The vendor driver is no longer seeing the value the user provided, > what if some future vendor structure ends with something like: > > __u32 nr_entries; > __u32 entries[]; > } > > The core code clobbered the user value, so the vendor driver wouldn't > be able to perform any additional user copies. Clearly that's also a > bug that could be fixed at the time such functionality becomes > necessary, it seems unnecessary to perform that clobbering in the > first place if everyone is on guard for user supplied data. Thanks, Sorry I missed this in my last reply. Make sense not to clobber user data in the core. So we should pass the argsz as is to the vendor driver, but copy from user only up to the size of the current kernel supports. i.e. /* * User might be using a newer UAPI header which has a larger data * size, we shall support the existing flags within the current * size. Copy the remaining user data _after_ minsz but not more * than the current kernel supported size. */ if (copy_from_user((void *)_info + minsz, uinfo + minsz, min(inv_info.argsz, maxsz) - minsz)) return -EFAULT; Currently, vendor driver does not handle user pointer. Once the extension as you described above becomes necessary, we can change the vendor driver interface. Is that a reasonable plan?
Re: [PATCH v4 1/5] docs: IOMMU user API
On Tue, 14 Jul 2020 13:04:12 -0600 Alex Williamson wrote: > On Mon, 13 Jul 2020 22:00:23 -0700 > Jacob Pan wrote: > > > Hi Alex, > > > > On Mon, 13 Jul 2020 16:48:42 -0600 > > Alex Williamson wrote: > > > > > On Tue, 7 Jul 2020 16:43:45 -0700 > > > Jacob Pan wrote: > > > > > > > IOMMU UAPI is newly introduced to support communications between > > > > guest virtual IOMMU and host IOMMU. There has been lots of > > > > discussions on how it should work with VFIO UAPI and userspace > > > > in general. > > > > > > > > This document is indended to clarify the UAPI design and usage. > > > > The mechenics of how future extensions should be achieved are > > > > also covered > > > > > > mechanics > > > > > will fix. > > > > > > in this documentation. > > > > > > > > Signed-off-by: Liu Yi L > > > > Signed-off-by: Jacob Pan > > > > --- > > > > Documentation/userspace-api/iommu.rst | 312 > > > > ++ 1 file changed, 312 > > > > insertions(+) create mode 100644 > > > > Documentation/userspace-api/iommu.rst > > > > > > > > diff --git a/Documentation/userspace-api/iommu.rst > > > > b/Documentation/userspace-api/iommu.rst new file mode 100644 > > > > index ..581b462c2cec > > > > --- /dev/null > > > > +++ b/Documentation/userspace-api/iommu.rst > > > > @@ -0,0 +1,312 @@ > > > > +.. SPDX-License-Identifier: GPL-2.0 > > > > +.. iommu: > > > > + > > > > += > > > > +IOMMU Userspace API > > > > += > > > > + > > > > +IOMMU UAPI is used for virtualization cases where > > > > communications are +needed between physical and virtual IOMMU > > > > drivers. For native +usage, IOMMU is a system device which does > > > > not need to communicate +with user space directly. > > > > + > > > > +The primary use cases are guest Shared Virtual Address (SVA) > > > > and +guest IO virtual address (IOVA), wherein a virtual IOMMU > > > > (vIOMMU) is +required to communicate with the physical IOMMU in > > > > the host. + > > > > +.. contents:: :local: > > > > + > > > > +Functionalities > > > > +=== > > > > +Communications of user and kernel involve both directions. The > > > > +supported user-kernel APIs are as follows: > > > > + > > > > +1. Alloc/Free PASID > > > > +2. Bind/unbind guest PASID (e.g. Intel VT-d) > > > > +3. Bind/unbind guest PASID table (e.g. ARM sMMU) > > > > +4. Invalidate IOMMU caches > > > > +5. Service page requests > > > > + > > > > +Requirements > > > > + > > > > +The IOMMU UAPIs are generic and extensible to meet the > > > > following +requirements: > > > > + > > > > +1. Emulated and para-virtualised vIOMMUs > > > > +2. Multiple vendors (Intel VT-d, ARM sMMU, etc.) > > > > +3. Extensions to the UAPI shall not break existing user space > > > > + > > > > +Interfaces > > > > +== > > > > +Although the data structures defined in IOMMU UAPI are > > > > self-contained, +there is no user API functions introduced. > > > > Instead, IOMMU UAPI is +designed to work with existing user > > > > driver frameworks such as VFIO. + > > > > +Extension Rules & Precautions > > > > +- > > > > +When IOMMU UAPI gets extended, the data structures can *only* > > > > be +modified in two ways: > > > > + > > > > +1. Adding new fields by re-purposing the padding[] field. No > > > > size change. +2. Adding new union members at the end. May > > > > increase in size. + > > > > +No new fields can be added *after* the variable sized union in > > > > that it +will break backward compatibility when offset moves. In > > > > both cases, a +new flag must be accompanied with a new field > > > > such that the IOMMU +driver can process the data based on the > > > > new flag. Version field is +only reserved for the unlikely > > > > event of UAPI upgrade at its entirety. + > > > > +It's *always* the caller's responsibility to indicate the size > > > > of the +structure passed by setting argsz appropriately. > > > > +Though at the same time, argsz is user provided data which is > > > > not +trusted. The argsz field allows the user to indicate how > > > > much data +they're providing, it's still the kernel's > > > > responsibility to validate +whether it's correct and sufficient > > > > for the requested operation. + > > > > +Compatibility Checking > > > > +-- > > > > +When IOMMU UAPI extension results in size increase, user such > > > > as VFIO +has to handle the following cases: > > > > + > > > > +1. User and kernel has exact size match > > > > +2. An older user with older kernel header (smaller UAPI size) > > > > running on a > > > > + newer kernel (larger UAPI size) > > > > +3. A newer user with newer kernel header (larger UAPI size) > > > > running > > > > + on an older kernel. > > > > +4. A malicious/misbehaving user pass illegal/invalid size but > > > > within > > > > + range. The data may contain garbage. > > > > > > I'm
Re: [PATCH v4 1/5] docs: IOMMU user API
On Mon, 13 Jul 2020 22:00:23 -0700 Jacob Pan wrote: > Hi Alex, > > On Mon, 13 Jul 2020 16:48:42 -0600 > Alex Williamson wrote: > > > On Tue, 7 Jul 2020 16:43:45 -0700 > > Jacob Pan wrote: > > > > > IOMMU UAPI is newly introduced to support communications between > > > guest virtual IOMMU and host IOMMU. There has been lots of > > > discussions on how it should work with VFIO UAPI and userspace in > > > general. > > > > > > This document is indended to clarify the UAPI design and usage. The > > > mechenics of how future extensions should be achieved are also > > > covered > > > > mechanics > > > will fix. > > > > in this documentation. > > > > > > Signed-off-by: Liu Yi L > > > Signed-off-by: Jacob Pan > > > --- > > > Documentation/userspace-api/iommu.rst | 312 > > > ++ 1 file changed, 312 insertions(+) > > > create mode 100644 Documentation/userspace-api/iommu.rst > > > > > > diff --git a/Documentation/userspace-api/iommu.rst > > > b/Documentation/userspace-api/iommu.rst new file mode 100644 > > > index ..581b462c2cec > > > --- /dev/null > > > +++ b/Documentation/userspace-api/iommu.rst > > > @@ -0,0 +1,312 @@ > > > +.. SPDX-License-Identifier: GPL-2.0 > > > +.. iommu: > > > + > > > += > > > +IOMMU Userspace API > > > += > > > + > > > +IOMMU UAPI is used for virtualization cases where communications > > > are +needed between physical and virtual IOMMU drivers. For native > > > +usage, IOMMU is a system device which does not need to communicate > > > +with user space directly. > > > + > > > +The primary use cases are guest Shared Virtual Address (SVA) and > > > +guest IO virtual address (IOVA), wherein a virtual IOMMU (vIOMMU) > > > is +required to communicate with the physical IOMMU in the host. > > > + > > > +.. contents:: :local: > > > + > > > +Functionalities > > > +=== > > > +Communications of user and kernel involve both directions. The > > > +supported user-kernel APIs are as follows: > > > + > > > +1. Alloc/Free PASID > > > +2. Bind/unbind guest PASID (e.g. Intel VT-d) > > > +3. Bind/unbind guest PASID table (e.g. ARM sMMU) > > > +4. Invalidate IOMMU caches > > > +5. Service page requests > > > + > > > +Requirements > > > + > > > +The IOMMU UAPIs are generic and extensible to meet the following > > > +requirements: > > > + > > > +1. Emulated and para-virtualised vIOMMUs > > > +2. Multiple vendors (Intel VT-d, ARM sMMU, etc.) > > > +3. Extensions to the UAPI shall not break existing user space > > > + > > > +Interfaces > > > +== > > > +Although the data structures defined in IOMMU UAPI are > > > self-contained, +there is no user API functions introduced. > > > Instead, IOMMU UAPI is +designed to work with existing user driver > > > frameworks such as VFIO. + > > > +Extension Rules & Precautions > > > +- > > > +When IOMMU UAPI gets extended, the data structures can *only* be > > > +modified in two ways: > > > + > > > +1. Adding new fields by re-purposing the padding[] field. No size > > > change. +2. Adding new union members at the end. May increase in > > > size. + > > > +No new fields can be added *after* the variable sized union in > > > that it +will break backward compatibility when offset moves. In > > > both cases, a +new flag must be accompanied with a new field such > > > that the IOMMU +driver can process the data based on the new flag. > > > Version field is +only reserved for the unlikely event of UAPI > > > upgrade at its entirety. + > > > +It's *always* the caller's responsibility to indicate the size of > > > the +structure passed by setting argsz appropriately. > > > +Though at the same time, argsz is user provided data which is not > > > +trusted. The argsz field allows the user to indicate how much data > > > +they're providing, it's still the kernel's responsibility to > > > validate +whether it's correct and sufficient for the requested > > > operation. + > > > +Compatibility Checking > > > +-- > > > +When IOMMU UAPI extension results in size increase, user such as > > > VFIO +has to handle the following cases: > > > + > > > +1. User and kernel has exact size match > > > +2. An older user with older kernel header (smaller UAPI size) > > > running on a > > > + newer kernel (larger UAPI size) > > > +3. A newer user with newer kernel header (larger UAPI size) running > > > + on an older kernel. > > > +4. A malicious/misbehaving user pass illegal/invalid size but > > > within > > > + range. The data may contain garbage. > > > > I'm still not sure where VFIO has responsibility in managing any of > > these cases. I think we've determined that VFIO is just the wrapper > > and call-through mechanism, it's the UAPI core implementation and > > IOMMU drivers that are responsible for this. > > > That is right, I shall rewrite the responsibility to be held by
Re: [PATCH v4 1/5] docs: IOMMU user API
Hi Alex, On Mon, 13 Jul 2020 16:48:42 -0600 Alex Williamson wrote: > On Tue, 7 Jul 2020 16:43:45 -0700 > Jacob Pan wrote: > > > IOMMU UAPI is newly introduced to support communications between > > guest virtual IOMMU and host IOMMU. There has been lots of > > discussions on how it should work with VFIO UAPI and userspace in > > general. > > > > This document is indended to clarify the UAPI design and usage. The > > mechenics of how future extensions should be achieved are also > > covered > > mechanics > will fix. > > in this documentation. > > > > Signed-off-by: Liu Yi L > > Signed-off-by: Jacob Pan > > --- > > Documentation/userspace-api/iommu.rst | 312 > > ++ 1 file changed, 312 insertions(+) > > create mode 100644 Documentation/userspace-api/iommu.rst > > > > diff --git a/Documentation/userspace-api/iommu.rst > > b/Documentation/userspace-api/iommu.rst new file mode 100644 > > index ..581b462c2cec > > --- /dev/null > > +++ b/Documentation/userspace-api/iommu.rst > > @@ -0,0 +1,312 @@ > > +.. SPDX-License-Identifier: GPL-2.0 > > +.. iommu: > > + > > += > > +IOMMU Userspace API > > += > > + > > +IOMMU UAPI is used for virtualization cases where communications > > are +needed between physical and virtual IOMMU drivers. For native > > +usage, IOMMU is a system device which does not need to communicate > > +with user space directly. > > + > > +The primary use cases are guest Shared Virtual Address (SVA) and > > +guest IO virtual address (IOVA), wherein a virtual IOMMU (vIOMMU) > > is +required to communicate with the physical IOMMU in the host. > > + > > +.. contents:: :local: > > + > > +Functionalities > > +=== > > +Communications of user and kernel involve both directions. The > > +supported user-kernel APIs are as follows: > > + > > +1. Alloc/Free PASID > > +2. Bind/unbind guest PASID (e.g. Intel VT-d) > > +3. Bind/unbind guest PASID table (e.g. ARM sMMU) > > +4. Invalidate IOMMU caches > > +5. Service page requests > > + > > +Requirements > > + > > +The IOMMU UAPIs are generic and extensible to meet the following > > +requirements: > > + > > +1. Emulated and para-virtualised vIOMMUs > > +2. Multiple vendors (Intel VT-d, ARM sMMU, etc.) > > +3. Extensions to the UAPI shall not break existing user space > > + > > +Interfaces > > +== > > +Although the data structures defined in IOMMU UAPI are > > self-contained, +there is no user API functions introduced. > > Instead, IOMMU UAPI is +designed to work with existing user driver > > frameworks such as VFIO. + > > +Extension Rules & Precautions > > +- > > +When IOMMU UAPI gets extended, the data structures can *only* be > > +modified in two ways: > > + > > +1. Adding new fields by re-purposing the padding[] field. No size > > change. +2. Adding new union members at the end. May increase in > > size. + > > +No new fields can be added *after* the variable sized union in > > that it +will break backward compatibility when offset moves. In > > both cases, a +new flag must be accompanied with a new field such > > that the IOMMU +driver can process the data based on the new flag. > > Version field is +only reserved for the unlikely event of UAPI > > upgrade at its entirety. + > > +It's *always* the caller's responsibility to indicate the size of > > the +structure passed by setting argsz appropriately. > > +Though at the same time, argsz is user provided data which is not > > +trusted. The argsz field allows the user to indicate how much data > > +they're providing, it's still the kernel's responsibility to > > validate +whether it's correct and sufficient for the requested > > operation. + > > +Compatibility Checking > > +-- > > +When IOMMU UAPI extension results in size increase, user such as > > VFIO +has to handle the following cases: > > + > > +1. User and kernel has exact size match > > +2. An older user with older kernel header (smaller UAPI size) > > running on a > > + newer kernel (larger UAPI size) > > +3. A newer user with newer kernel header (larger UAPI size) running > > + on an older kernel. > > +4. A malicious/misbehaving user pass illegal/invalid size but > > within > > + range. The data may contain garbage. > > I'm still not sure where VFIO has responsibility in managing any of > these cases. I think we've determined that VFIO is just the wrapper > and call-through mechanism, it's the UAPI core implementation and > IOMMU drivers that are responsible for this. > That is right, I shall rewrite the responsibility to be held by IOMMU core. "When IOMMU UAPI extension results in size increase, IOMMU UAPI core shall handle the following cases:" > > + > > +Feature Checking > > + > > +While launching a guest with vIOMMU, it is important to ensure > > that host +can support the UAPI data structures to be used for > >
Re: [PATCH v4 1/5] docs: IOMMU user API
On Tue, 7 Jul 2020 16:43:45 -0700 Jacob Pan wrote: > IOMMU UAPI is newly introduced to support communications between guest > virtual IOMMU and host IOMMU. There has been lots of discussions on how > it should work with VFIO UAPI and userspace in general. > > This document is indended to clarify the UAPI design and usage. The > mechenics of how future extensions should be achieved are also covered mechanics > in this documentation. > > Signed-off-by: Liu Yi L > Signed-off-by: Jacob Pan > --- > Documentation/userspace-api/iommu.rst | 312 > ++ > 1 file changed, 312 insertions(+) > create mode 100644 Documentation/userspace-api/iommu.rst > > diff --git a/Documentation/userspace-api/iommu.rst > b/Documentation/userspace-api/iommu.rst > new file mode 100644 > index ..581b462c2cec > --- /dev/null > +++ b/Documentation/userspace-api/iommu.rst > @@ -0,0 +1,312 @@ > +.. SPDX-License-Identifier: GPL-2.0 > +.. iommu: > + > += > +IOMMU Userspace API > += > + > +IOMMU UAPI is used for virtualization cases where communications are > +needed between physical and virtual IOMMU drivers. For native > +usage, IOMMU is a system device which does not need to communicate > +with user space directly. > + > +The primary use cases are guest Shared Virtual Address (SVA) and > +guest IO virtual address (IOVA), wherein a virtual IOMMU (vIOMMU) is > +required to communicate with the physical IOMMU in the host. > + > +.. contents:: :local: > + > +Functionalities > +=== > +Communications of user and kernel involve both directions. The > +supported user-kernel APIs are as follows: > + > +1. Alloc/Free PASID > +2. Bind/unbind guest PASID (e.g. Intel VT-d) > +3. Bind/unbind guest PASID table (e.g. ARM sMMU) > +4. Invalidate IOMMU caches > +5. Service page requests > + > +Requirements > + > +The IOMMU UAPIs are generic and extensible to meet the following > +requirements: > + > +1. Emulated and para-virtualised vIOMMUs > +2. Multiple vendors (Intel VT-d, ARM sMMU, etc.) > +3. Extensions to the UAPI shall not break existing user space > + > +Interfaces > +== > +Although the data structures defined in IOMMU UAPI are self-contained, > +there is no user API functions introduced. Instead, IOMMU UAPI is > +designed to work with existing user driver frameworks such as VFIO. > + > +Extension Rules & Precautions > +- > +When IOMMU UAPI gets extended, the data structures can *only* be > +modified in two ways: > + > +1. Adding new fields by re-purposing the padding[] field. No size change. > +2. Adding new union members at the end. May increase in size. > + > +No new fields can be added *after* the variable sized union in that it > +will break backward compatibility when offset moves. In both cases, a > +new flag must be accompanied with a new field such that the IOMMU > +driver can process the data based on the new flag. Version field is > +only reserved for the unlikely event of UAPI upgrade at its entirety. > + > +It's *always* the caller's responsibility to indicate the size of the > +structure passed by setting argsz appropriately. > +Though at the same time, argsz is user provided data which is not > +trusted. The argsz field allows the user to indicate how much data > +they're providing, it's still the kernel's responsibility to validate > +whether it's correct and sufficient for the requested operation. > + > +Compatibility Checking > +-- > +When IOMMU UAPI extension results in size increase, user such as VFIO > +has to handle the following cases: > + > +1. User and kernel has exact size match > +2. An older user with older kernel header (smaller UAPI size) running on a > + newer kernel (larger UAPI size) > +3. A newer user with newer kernel header (larger UAPI size) running > + on an older kernel. > +4. A malicious/misbehaving user pass illegal/invalid size but within > + range. The data may contain garbage. I'm still not sure where VFIO has responsibility in managing any of these cases. I think we've determined that VFIO is just the wrapper and call-through mechanism, it's the UAPI core implementation and IOMMU drivers that are responsible for this. > + > +Feature Checking > + > +While launching a guest with vIOMMU, it is important to ensure that host > +can support the UAPI data structures to be used for vIOMMU-pIOMMU > +communications. Without upfront compatibility checking, future faults > +are difficult to report even in normal conditions. For example, TLB > +invalidations should always succeed. There is no architectural way to > +report back to the vIOMMU if the UAPI data is incompatible. If that > +happens, in order to protect IOMMU iosolation guarantee, we have to > +resort to not giving completion status in vIOMMU. This may result in > +VM hang. > + > +For this reason the following IOMMU UAPIs cannot
Re: [PATCH v4 1/5] docs: IOMMU user API
Hi Jacob, On 7/8/20 11:29 PM, Jacob Pan wrote: On Wed, 8 Jul 2020 10:07:13 +0800 Lu Baolu wrote: Hi, On 7/8/20 7:43 AM, Jacob Pan wrote: +For UAPIs that are shared with in-kernel users, a wrapper function +is provided to distinguish the callers. For example, + +Userspace caller :: + + int iommu_sva_unbind_gpasid(struct iommu_domain *domain, struct device *dev, + void __user *udata) + +In-kernel caller :: + + int __iommu_sva_unbind_gpasid(struct iommu_domain *domain, struct device *dev, + struct iommu_gpasid_bind_data *data) iommu_page_response() may have the same needs. Can we reach an agreement on the naming rules? Yes iommu_page_response() also has to deal with in-kernel and UAPI callers. I left it out because I thought the current VFIO and SVA common code is not ready for PRQ yet, I might be reading old news :) argsz need to be handled as well. Perhaps we can wait until this set is settled? Do you have a suggestion on the naming rules? I have no suggestion on the naming rules, just wanted to check whether others have any preference. I agree that we can wait until this series is settled. Best regards, baolu
Re: [PATCH v4 1/5] docs: IOMMU user API
On Wed, 8 Jul 2020 10:07:13 +0800 Lu Baolu wrote: > Hi, > > On 7/8/20 7:43 AM, Jacob Pan wrote: > > +For UAPIs that are shared with in-kernel users, a wrapper function > > +is provided to distinguish the callers. For example, > > + > > +Userspace caller :: > > + > > + int iommu_sva_unbind_gpasid(struct iommu_domain *domain, struct > > device *dev, > > + void __user *udata) > > + > > +In-kernel caller :: > > + > > + int __iommu_sva_unbind_gpasid(struct iommu_domain *domain, > > struct device *dev, > > + struct iommu_gpasid_bind_data *data) > > iommu_page_response() may have the same needs. Can we reach an > agreement on the naming rules? > Yes iommu_page_response() also has to deal with in-kernel and UAPI callers. I left it out because I thought the current VFIO and SVA common code is not ready for PRQ yet, I might be reading old news :) argsz need to be handled as well. Perhaps we can wait until this set is settled? Do you have a suggestion on the naming rules? > Best regards, > baolu [Jacob Pan]
Re: [PATCH v4 1/5] docs: IOMMU user API
Hi, On 7/8/20 7:43 AM, Jacob Pan wrote: +For UAPIs that are shared with in-kernel users, a wrapper function +is provided to distinguish the callers. For example, + +Userspace caller :: + + int iommu_sva_unbind_gpasid(struct iommu_domain *domain, struct device *dev, + void __user *udata) + +In-kernel caller :: + + int __iommu_sva_unbind_gpasid(struct iommu_domain *domain, struct device *dev, + struct iommu_gpasid_bind_data *data) iommu_page_response() may have the same needs. Can we reach an agreement on the naming rules? Best regards, baolu
[PATCH v4 1/5] docs: IOMMU user API
IOMMU UAPI is newly introduced to support communications between guest virtual IOMMU and host IOMMU. There has been lots of discussions on how it should work with VFIO UAPI and userspace in general. This document is indended to clarify the UAPI design and usage. The mechenics of how future extensions should be achieved are also covered in this documentation. Signed-off-by: Liu Yi L Signed-off-by: Jacob Pan --- Documentation/userspace-api/iommu.rst | 312 ++ 1 file changed, 312 insertions(+) create mode 100644 Documentation/userspace-api/iommu.rst diff --git a/Documentation/userspace-api/iommu.rst b/Documentation/userspace-api/iommu.rst new file mode 100644 index ..581b462c2cec --- /dev/null +++ b/Documentation/userspace-api/iommu.rst @@ -0,0 +1,312 @@ +.. SPDX-License-Identifier: GPL-2.0 +.. iommu: + += +IOMMU Userspace API += + +IOMMU UAPI is used for virtualization cases where communications are +needed between physical and virtual IOMMU drivers. For native +usage, IOMMU is a system device which does not need to communicate +with user space directly. + +The primary use cases are guest Shared Virtual Address (SVA) and +guest IO virtual address (IOVA), wherein a virtual IOMMU (vIOMMU) is +required to communicate with the physical IOMMU in the host. + +.. contents:: :local: + +Functionalities +=== +Communications of user and kernel involve both directions. The +supported user-kernel APIs are as follows: + +1. Alloc/Free PASID +2. Bind/unbind guest PASID (e.g. Intel VT-d) +3. Bind/unbind guest PASID table (e.g. ARM sMMU) +4. Invalidate IOMMU caches +5. Service page requests + +Requirements + +The IOMMU UAPIs are generic and extensible to meet the following +requirements: + +1. Emulated and para-virtualised vIOMMUs +2. Multiple vendors (Intel VT-d, ARM sMMU, etc.) +3. Extensions to the UAPI shall not break existing user space + +Interfaces +== +Although the data structures defined in IOMMU UAPI are self-contained, +there is no user API functions introduced. Instead, IOMMU UAPI is +designed to work with existing user driver frameworks such as VFIO. + +Extension Rules & Precautions +- +When IOMMU UAPI gets extended, the data structures can *only* be +modified in two ways: + +1. Adding new fields by re-purposing the padding[] field. No size change. +2. Adding new union members at the end. May increase in size. + +No new fields can be added *after* the variable sized union in that it +will break backward compatibility when offset moves. In both cases, a +new flag must be accompanied with a new field such that the IOMMU +driver can process the data based on the new flag. Version field is +only reserved for the unlikely event of UAPI upgrade at its entirety. + +It's *always* the caller's responsibility to indicate the size of the +structure passed by setting argsz appropriately. +Though at the same time, argsz is user provided data which is not +trusted. The argsz field allows the user to indicate how much data +they're providing, it's still the kernel's responsibility to validate +whether it's correct and sufficient for the requested operation. + +Compatibility Checking +-- +When IOMMU UAPI extension results in size increase, user such as VFIO +has to handle the following cases: + +1. User and kernel has exact size match +2. An older user with older kernel header (smaller UAPI size) running on a + newer kernel (larger UAPI size) +3. A newer user with newer kernel header (larger UAPI size) running + on an older kernel. +4. A malicious/misbehaving user pass illegal/invalid size but within + range. The data may contain garbage. + +Feature Checking + +While launching a guest with vIOMMU, it is important to ensure that host +can support the UAPI data structures to be used for vIOMMU-pIOMMU +communications. Without upfront compatibility checking, future faults +are difficult to report even in normal conditions. For example, TLB +invalidations should always succeed. There is no architectural way to +report back to the vIOMMU if the UAPI data is incompatible. If that +happens, in order to protect IOMMU iosolation guarantee, we have to +resort to not giving completion status in vIOMMU. This may result in +VM hang. + +For this reason the following IOMMU UAPIs cannot fail: + +1. Free PASID +2. Unbind guest PASID +3. Unbind guest PASID table (SMMU) +4. Cache invalidate + +User applications such as QEMU is expected to import kernel UAPI +headers. Backward compatibility is supported per feature flags. +For example, an older QEMU (with older kernel header) can run on newer +kernel. Newer QEMU (with new kernel header) may refuse to initialize +on an older kernel if new feature flags are not supported by older +kernel. Simply recompile existing code with newer kernel header should +not be an issue in that
