On 07/06/2019 03:24, Prakhya, Sai Praneeth wrote:
Hi All,
I am working on an IOMMU driver feature that allows a user to specify if the DMA from a device
should be translated by IOMMU or not. Presently, we support only all devices or none mode i.e. if
user specifies "iommu=pt" [X86] or "iommu.passthrough" [ARM64] through kernel
command line, all the devices would be in pass through mode and we don't have per device
granularity, but, we were requested by a customer to selectively put devices in pass through mode
and not all.
It's interesting to see this from a fresh angle which isn't clouded by
other SoC GPU details - thanks for the proposal! A couple more thoughts
jump out immediately...
Since, this feature could be generic across architectures, we thought it would
be better if the user interface is discussed in the community first. We are
envisioning this to be used both during boot time and runtime and hence having
a kernel command line argument along with a sysfs entry are needed. So, please
pour in your suggestions on how the user interface should look like to make it
architecture agnostic.
1. Have a kernel command line argument that takes a list of BDF's as an
input and puts them in pass through mode
a. Accepting BDF as an input has a downside - BDF is dynamic and could
change if BIOS/OS enumerates a new device in next reboot
b. Accepting <vendor_id:device_id> pair as an input has a downside - What
to do when there are multiple such devices and user would like to put only some of
them in PT mode
c. Not all devices are PCI in the first place.
2. Have a sysfs file which takes 1 or 0 as an input to enable/disable pass
through mode. Some places that seem to be reasonable are
a. /sys/class/iommu/dmar0/devices/
b. /sys/kernel/iommu_groups/<id>/devices
Note that this this works out a bit tricky to actually use, since
changing the meaning of DMA addresses under the device's feet would be
disastrous. It can only safely take effect by unbinding and rebinding
the driver and/or resetting the device (as a side note, this starts to
overlap with the IOMMU-drivers-as-modules concept, and whatever solution
to this we end up with may be helpful in making that work too). In most
cases that's probably viable, but not every driver supports unbinding,
and either way what if the device in question is the one hosting the
root filesystem... :/
Robin.
I am looking for a consensus on *how the kernel command line argument should
look like and path for sysfs entry*. Also, please note that if a device is put
in pass through mode it won't be available for the guest and that's ok.
Regards,
Sai
PS: Idea credits: Ashok Raj
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