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Re: [PATCH 0/3] kvm: fix two svm pmu virtualization bugs

From: Like Xu
Subject: Re: [PATCH 0/3] kvm: fix two svm pmu virtualization bugs
Date: Mon, 21 Nov 2022 14:42:20 +0800
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.5.0

On 19/11/2022 8:28 pm, Dongli Zhang wrote:
This patchset is to fix two svm pmu virtualization bugs.

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 ...

Many QEMU vendor forks already have similar fixes, and
thanks for bringing this issue back to the mainline.

[    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 patch 1-2 is to disable the pmu virtualization via KVM_PMU_CAP_DISABLE
if the per-vcpu "pmu" property is disabled.

I considered 'KVM_X86_SET_MSR_FILTER' initially.
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.

I'm not sure if you're saying KVM doing something wrong, I don't think so
because KVM doesn't sense the system_reset defined by QEME or other user space,
AMD's vPMC will continue to be enabled (if it was enabled before), generating 
injection into the guest, and the newly started guest doesn't realize the counter is still
enabled and blowing up the error log.

(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 patch 3 is to fix the issue by resetting AMD pmu registers as well as
Intel registers.

This fix idea looks good, it does require syncing the new changed device state of QEMU to KVM.

This patchset does cover 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

Dongli Zhang (3):
       kvm: introduce a helper before creating the 1st vcpu
       i386: kvm: disable KVM_CAP_PMU_CAPABILITY if "pmu" is disabled
       target/i386/kvm: get and put AMD pmu registers

  accel/kvm/kvm-all.c    |   7 ++-
  include/sysemu/kvm.h   |   2 +
  target/arm/kvm64.c     |   4 ++
  target/i386/cpu.h      |   5 +++
  target/i386/kvm/kvm.c  | 104 +++++++++++++++++++++++++++++++++++++++++++-
  target/mips/kvm.c      |   4 ++
  target/ppc/kvm.c       |   4 ++
  target/riscv/kvm.c     |   4 ++
  target/s390x/kvm/kvm.c |   4 ++
  9 files changed, 134 insertions(+), 4 deletions(-)

Thank you very much!

Dongli Zhang

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