Re: [PATCH 0/5] KVM: x86/pmu: Hide guest counter updates from the VMRUN instruction

Like Xu
2023-03-23T08:16:21Z

On 13/3/2023 6:57 pm, Sandipan Das wrote:
> +CC: Santosh, Tom, Ananth
>
> Hi Like,
>
> On 3/10/2023 4:23 PM, Like Xu wrote:
>> Considering that developers are more likely to have access to AMD VMs
>> and use vPMU inside guest, there's a dark cloud that needs to rain.
>> The x86_64/pmu_event_filter_test always fails on Zen3 boxes:
>>
>> test_amd_deny_list: Branch instructions retired = 43 (expected 42)
>> test_without_filter: Branch instructions retired = 43 (expected 42)
>> test_member_allow_list: Branch instructions retired = 43 (expected 42)
>> test_not_member_deny_list: Branch instructions retired = 43 (expected 42)
>>
>> ,which is not caused by the event_filter feature (otherwise it's zero).
>>
>> After some dubious guessing and microtesting on Zen3+ pmu hardware,
>> we found that VMRUN or one of the instructions in __svm_vcpu_run()
>> causes a guest-only enabled counter for counting guest instruction (in the
>> pmu_event_filter case, the branch instruction) to always increase by one
>> right after each vm_entry.
>>
>> This creates an inconsistency with the AMD64_EVENTSEL_GUESTONLY,
>> where the vPMU user in the VM does not expect to see any counter
>> changes due to the SVM transaction at all. This patch set provides a low
>> overhead software fix until HW change arrives or simply no fix planned.
>>
>
> Yes, VMRUNs do get counted as retired branches in the guest context. My
> understanding is that this behaviour applies to all generations of Zen
> and even some older ones too, not just Zen 3 and later. I also do not
> expect this to change in the near future.
>
> - Sandipan
>

Interesting, thanks for confirming this issue (I presume this is an official reply).

If this hardware behavior is not expected to change, this software-based correction
would be essential and urgent for those who only use vPMU inside AMD guests.

Let's see what will happen ...