** Changed in: linux (Ubuntu Jammy)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1853306
Title:
[22.04 FEAT] Enhanced Interpretation for PCI Functions on s390x -
kernel part
Status in Ubuntu on IBM z Systems:
Fix Released
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Jammy:
Fix Committed
Status in linux source package in Kinetic:
Won't Fix
Status in linux source package in Lunar:
Fix Released
Status in linux source package in Mantic:
Fix Released
Bug description:
[ Impact ]
* Currently the PCI passthrough implementation for s390x is based on
intercepting PCI I/O instructions, which leads to a reduced I/O performance
compared to the execution of PCI instructions directly in LPAR.
* Hence users may face I/O bottlenecks when using PCI devices in passthrough
mode based on the current implementation.
* For avoiding this and to improve performance, the interpretive execution
of the PCI store and PCI load instructions get enabled.
* A further improvement is achieved by enabling the
Adapter-Event-Notification
Interpretation (AENI).
* Since LTS releases are the main focus for stable and long running KVM
workloads, it is highly desired to get this backported to the jammy kernel
(and because the next LTS is still some time away).
[ Test Plan ]
* Have an Ubuntu Server 22.04 installation on LPAR,
that is able to access (ideally multiple) PCI devices,
like RoCE Express (network) or NVMe disks.
* Setup KVM and pass through (ideally multiple) of these PCI devices
(that are otherwise unused on the KVM host).
* Generate IO load on these passed through PCI devices,
for example with stress-ng, using class network and/or device
and/or io stressors.
* This PR also introduces a new kernel config option 'VFIO_PCI_ZDEV_KVM'
that allows to enable support for the s390x-specific extensions and
enhancements to KVM passthrough, such as interpretive execution of zPCI
instructions and is with this PR and got enabled.
* The qemu autopkgtest (also needed due to LP#1853307) will be a got fit
to identify any potential regressions, also in the kvm kernel area.
* zPCI passthrough related test will be done by IBM.
[ Where problems could occur ]
* The modifications do not change the way users or APIs have to make
use of PCI passthrough, only the internal implementation got modified.
* The vast majority of the code changes/or additional code is s390x-specific,
under arch/s390 and drivers/s390.
* However there is also common code touched:
* 'kvm: use kvfree() in kvm_arch_free_vm()' touches
arch/arm64/include/asm/kvm_host.h, arch/arm64/kvm/arm.c,
arch/x86/include/asm/kvm_host.h, arch/x86/kvm/x86.c,
include/linux/kvm_host.h switches in kvm_arch_free_vm() from kfree() to
kvfree() allowing to use the common variant, which is upstream since v5.16
and with that well established.
* And 'vfio-pci/zdev: add open/close device hooks' touches
drivers/vfio/pci/vfio_pci_core.c and drivers/vfio/pci/vfio_pci_zdev.c
include/linux/vfio_pci_core.h add now code to introduce device hooks.
It's upstream since kernel 6.0.
* 'KVM: s390: pci: provide routines for en-/disabling interrupt forwarding'
expands a single #if statement in include/linux/sched/user.h.
* 'KVM: s390: add KVM_S390_ZPCI_OP to manage guest zPCI devices'
adds s390x specific KVM_S390_ZPCI_OP and it's definition to
include/uapi/linux/kvm.h.
* And 'vfio-pci/zdev: different maxstbl for interpreted devices' and
'vfio-pci/zdev: add function handle to clp base capability' expand
s390x-specific (aka z-specific aka zdev) device structs in
include/uapi/linux/vfio_zdev.h.
* This shows that the vast majority of modifications are s390x specific,
even in most of the common code files.
* The remaining modifications in the (generally) common code files are
related to the newly introduced kernel option 'CONFIG_VFIO_PCI_ZDEV_KVM'
and documentation.
* The s390x changes are more significant, and could not only harm
passthrough itself for zPCI devices, but also KVM virtualization in
general.
* In addition to these kernel changes, qemu modifications are needed
as well (that are addressed at LP#1853307), this modified kernel
must be tested in combination with the updated qemu package.
- The qemu autopkgtest will be a got fit to identify any regressions,
also in the kernel.
- In addition some passthrough related test will be done by IBM
__________
The PCI Passthrough implementation is based on intercepting PCI I/O
instructions which leads to a reduced I/O performance compared to execution of
PCI instructions in LPAR.
For improved performance the interpretive execution of the PCI store and PCI
load instructions get enabled.
Further improvement is achieved by enabling the Adapter-Event-Notification
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