Re: [PATCH] KVM: x86/pmu: fix masking logic for MSR_CORE_PERF_GLOBAL_CTRL

[Liang, Kan]

On 2024-01-04 1:22 p.m., Sean Christopherson wrote:
> On Thu, Jan 04, 2024, Liang, Kan wrote:
>>
>>
>> On 2024-01-04 10:39 a.m., Paolo Bonzini wrote:
>>> When commit c59a1f106f5c ("KVM: x86/pmu: Add IA32_PEBS_ENABLE
>>> MSR emulation for extended PEBS") switched the initialization of
>>> cpuc->guest_switch_msrs to use compound literals, it screwed up
>>> the boolean logic:
>>>
>>> +	u64 pebs_mask = cpuc->pebs_enabled & x86_pmu.pebs_capable;
>>> ...
>>> -	arr[0].guest = intel_ctrl & ~cpuc->intel_ctrl_host_mask;
>>> -	arr[0].guest &= ~(cpuc->pebs_enabled & x86_pmu.pebs_capable);
>>> +               .guest = intel_ctrl & (~cpuc->intel_ctrl_host_mask | ~pebs_mask),
>>>
>>> Before the patch, the value of arr[0].guest would have been intel_ctrl &
>>> ~cpuc->intel_ctrl_host_mask & ~pebs_mask.  The intent is to always treat
>>> PEBS events as host-only because, while the guest runs, there is no way
>>> to tell the processor about the virtual address where to put PEBS records
>>> intended for the host.
>>>
>>> Unfortunately, the new expression can be expanded to
>>>
>>> 	(intel_ctrl & ~cpuc->intel_ctrl_host_mask) | (intel_ctrl & ~pebs_mask)
>>>
>>> which makes no sense; it includes any bit that isn't *both* marked as
>>> exclude_guest and using PEBS.  So, reinstate the old logic.  
>>
>> I think the old logic will completely disable the PEBS in guest
>> capability. Because the counter which is assigned to a guest PEBS event
>> will also be set in the pebs_mask. The old logic disable the counter in
>> GLOBAL_CTRL in guest. Nothing will be counted.
>>
>> Like once proposed a fix in the intel_guest_get_msrs().
>> https://lore.kernel.org/lkml/20231129095055.88060-1-likexu@xxxxxxxxxxx/
>> It should work for the issue.
> 
> No, that patch only affects the path where hardware supports enabling PEBS in the
> the guest, i.e. intel_guest_get_msrs() will bail before getting to that code due
> to the lack of x86_pmu.pebs_ept support, which IIUC is all pre-Icelake Intel CPUs.
> 
> 	if (!kvm_pmu || !x86_pmu.pebs_ept)
> 		return arr;
>

True, we have to disable all PEBS counters for pre-ICL as well.

I think what I missed is that the disable here is temporary. The
arr[global_ctrl].guest will be updated later for the x86_pmu.pebs_ept
platform, so the guest PEBS event should still work.

The patch looks good to me.

Reviewed-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>

Thanks,
Kan