Re: kvm guests crash when running "perf kvm top"

[Seth Forshee <sforshee@kernel.org>]

On Wed, Apr 09, 2025 at 10:05:00AM -0700, Sean Christopherson wrote:
> On Tue, Apr 08, 2025, Seth Forshee wrote:
> > A colleague of mine reported kvm guest hangs when running "perf kvm top"
> > with a 6.1 kernel. Initially it looked like the problem might be fixed
> > in newer kernels, but it turned out to be perf changes which must avoid
> > triggering the issue. I was able to reproduce the guest crashes with
> > 6.15-rc1 in both the host and the guest when using an older version of
> > perf. A bisect of perf landed on 7b100989b4f6 "perf evlist: Remove
> > __evlist__add_default", but this doesn't look to be fixing any kind of
> > issue like this.
> > 
> > This box has an Ice Lake CPU, and we can reproduce on other Ice Lakes
> > but could not reproduce on another box with Broadwell. On Broadwell
> > guests would crash with older kernels in the host, but this was fixed by
> > 971079464001 "KVM: x86/pmu: fix masking logic for
> > MSR_CORE_PERF_GLOBAL_CTRL". That does not fix the issues we see on Ice
> > Lake.
> > 
> > When the guests crash we aren't getting any output on the serial
> > console, but I got this from a memory dump:
> 
> ...
> 
> > Oops: 0000 [#1] PREEMPT SMP NOPTI
> > BUG: kernel NULL pointer dereference, address: 000000000000002828
> 
> FWIW, this is probably slightly corrupted.  When I run with EPT disabled, to force
> KVM to intercept #PFs, the reported CR2 is 0x28.  Which is consistent with the
> guest having DS_AREA=0.  I.e. the CPU is attempting to store into the DS/PEBS
> buffer.
> 
> As suspected, the issue is PEBS.  After adding a tracepoint to capture the MSRs
> that KVM loads as part of the perf transition, it's easy to see that PEBS_ENABLE
> gets loaded with a non-zero value immediate before death, doom, and destruction.
> 
>   CPU 0: kvm_entry: vcpu 0, rip 0xffffffff81000aa0 intr_info 0x80000b0e error_code 0x00000000
>   CPU 0: kvm_perf_msr: MSR 38f: host 1000f000000fe guest 1000f000000ff
>   CPU 0: kvm_perf_msr: MSR 600: host fffffe57186af000 guest 0
>   CPU 0: kvm_perf_msr: MSR 3f2: host 0 guest 0
>   CPU 0: kvm_perf_msr: MSR 3f1: host 0 guest 1
>   CPU 0: kvm_exit: vcpu 0 reason EXCEPTION_NMI rip 0xffffffff81000aa0 info1 0x0000000000000028 intr_info 0x80000b0e error_code 0x00000000
> 
> The underlying issue is that KVM's current PMU virtualization uses perf_events
> to proxy guest events, i.e. piggybacks intel_ctrl_guest_mask, which is also used
> by host userspace to communicate exclude_host/exclude_guest.  And so perf's
> intel_guest_get_msrs() allows using PEBS for guest events, but only if perf isn't
> using PEBS for host events.
> 
> I didn't actually verify that "perf kvm top" generates for events, but I assuming
> it's creating a precise, a.k.a. PEBS, event that measures _only_ guest, i.e.
> excludes host.  That causes a false positive of sorts in intel_guest_get_msrs(),
> and ultimately results in KVM running the guest with a PEBS event enabled, even
> though the guest isn't using the (virtual) PMU.
> 
> Pre-ICX CPUs don't isolate PEBS events across the guest/host boundary, and so
> perf/KVM hard disable PEBS on VM-Enter.  And a simple (well, simple for perf)
> precise event doesn't cause problems, because perf/KVM will disable PEBS events
> that are counting the host.  I.e. if a PEBS event counts host *and* guest, it's
> "fine".
> 
> Long story short, masking PEBS_ENABLE with the guest's value (in addition to
> what perf allows) fixes the issue on my end.  Assuming testing goes well, I'll
> post this as a proper patch.
> 
> --
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index cdb19e3ba3aa..1d01fb43a337 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -4336,7 +4336,7 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
>         arr[pebs_enable] = (struct perf_guest_switch_msr){
>                 .msr = MSR_IA32_PEBS_ENABLE,
>                 .host = cpuc->pebs_enabled & ~cpuc->intel_ctrl_guest_mask,
> -               .guest = pebs_mask & ~cpuc->intel_ctrl_host_mask,
> +               .guest = pebs_mask & ~cpuc->intel_ctrl_host_mask & kvm_pmu->pebs_enable,
>         };
>  
>         if (arr[pebs_enable].host) {

This fixes the issue for me, thanks!

> --
> 
> 
> > Let me know if I can provide any additional information or testing.
> 
> Uber nit: in the future, explicitly state whether a command is being run in the
> guest or host.  I had a brain fart and it took me an embarrasingly long time to
> grok that running "perf kvm top" in the guest would be nonsensical.

Apologies, I tried to make sure I differentiated between host vs guest
in my description since I know it gets confusing, but I missed that one.
I'll triple check for that in the future.