Re: [PATCH v2 0/2] target/i386/kvm: fix two svm pmu virtualization bugs

[Dongli Zhang]

Can I get feedback for this patchset, especially the [PATCH v2 2/2]?

About the [PATCH v2 2/2], currently the issue impacts the usage of PMUs on AMD
VM, especially the below case:

1. Enable panic on nmi.
2. Use perf to monitor the performance of VM. Although without a test, I think
the nmi watchdog has the same effect.
3. A sudden system reset, or a kernel panic (kdump/kexec).
4. After reboot, there will be random unknown NMI.
5. Unfortunately, the "panic on nmi" may panic the VM randomly at any time.

Thank you very much!

Dongli Zhang

On 12/1/22 16:22, Dongli Zhang wrote:
> This patchset is to fix two svm pmu virtualization bugs, x86 only.
> 
> version 1:
> https://lore.kernel.org/all/20221119122901.2469-1-dongli.zhang@oracle.com/
> 
> 1. The 1st bug is that "-cpu,-pmu" cannot disable svm pmu virtualization.
> 
> To use "-cpu EPYC" or "-cpu host,-pmu" cannot disable the pmu
> virtualization. There is still below at the VM linux side ...
> 
> [    0.510611] Performance Events: Fam17h+ core perfctr, AMD PMU driver.
> 
> ... although we expect something like below.
> 
> [    0.596381] Performance Events: PMU not available due to virtualization, 
> using software events only.
> [    0.600972] NMI watchdog: Perf NMI watchdog permanently disabled
> 
> The 1st patch has introduced a new x86 only accel/kvm property
> "pmu-cap-disabled=true" to disable the pmu virtualization via
> KVM_PMU_CAP_DISABLE.
> 
> I considered 'KVM_X86_SET_MSR_FILTER' initially before patchset v1.
> Since both KVM_X86_SET_MSR_FILTER and KVM_PMU_CAP_DISABLE are VM ioctl. I
> finally used the latter because it is easier to use.
> 
> 
> 2. The 2nd bug is that un-reclaimed perf events (after QEMU system_reset)
> at the KVM side may inject random unwanted/unknown NMIs to the VM.
> 
> The svm pmu registers are not reset during QEMU system_reset.
> 
> (1). The VM resets (e.g., via QEMU system_reset or VM kdump/kexec) while it
> is running "perf top". The pmu registers are not disabled gracefully.
> 
> (2). Although the x86_cpu_reset() resets many registers to zero, the
> kvm_put_msrs() does not puts AMD pmu registers to KVM side. As a result,
> some pmu events are still enabled at the KVM side.
> 
> (3). The KVM pmc_speculative_in_use() always returns true so that the events
> will not be reclaimed. The kvm_pmc->perf_event is still active.
> 
> (4). After the reboot, the VM kernel reports below error:
> 
> [    0.092011] Performance Events: Fam17h+ core perfctr, Broken BIOS 
> detected, complain to your hardware vendor.
> [    0.092023] [Firmware Bug]: the BIOS has corrupted hw-PMU resources (MSR 
> c0010200 is 530076)
> 
> (5). In a worse case, the active kvm_pmc->perf_event is still able to
> inject unknown NMIs randomly to the VM kernel.
> 
> [...] Uhhuh. NMI received for unknown reason 30 on CPU 0.
> 
> The 2nd patch is to fix the issue by resetting AMD pmu registers as well as
> Intel registers.
> 
> 
> This patchset does not cover PerfMonV2, until the below patchset is merged
> into the KVM side.
> 
> [PATCH v3 0/8] KVM: x86: Add AMD Guest PerfMonV2 PMU support
> https://lore.kernel.org/all/20221111102645.82001-1-likexu@tencent.com/
> 
> 
> Dongli Zhang (2):
>       target/i386/kvm: introduce 'pmu-cap-disabled' to set KVM_PMU_CAP_DISABLE
>       target/i386/kvm: get and put AMD pmu registers
> 
>  accel/kvm/kvm-all.c      |   1 +
>  include/sysemu/kvm_int.h |   1 +
>  qemu-options.hx          |   7 +++
>  target/i386/cpu.h        |   5 ++
>  target/i386/kvm/kvm.c    | 129 +++++++++++++++++++++++++++++++++++++++++-
>  5 files changed, 141 insertions(+), 2 deletions(-)
> 
> Thank you very much!
> 
> Dongli Zhang
> 
>