Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI and inject it to guest

Like Xu
2021-01-22T05:32:35Z

On 2021/1/16 1:42, Sean Christopherson wrote:
> On Fri, Jan 15, 2021, Xu, Like wrote:
>> On 2021/1/15 2:55, Sean Christopherson wrote:
>>> On Mon, Jan 04, 2021, Like Xu wrote:
>>>> + * Note: KVM disables the co-existence of guest PEBS and host PEBS.
>>> By "KVM", do you mean KVM's loading of the MSRs provided by intel_guest_get_msrs()?
>>> Because the PMU should really be the entity that controls guest vs. host. KVM
>>> should just be a dumb pipe that handles the mechanics of how values are context
>>> switch.
>>
>> The intel_guest_get_msrs() and atomic_switch_perf_msrs()
>> will work together to disable the co-existence of guest PEBS and host PEBS:
>>
>> https://lore.kernel.org/kvm/961e6135-ff6d-86d1-3b7b-a1846ad0e4c4@intel.com/
>>
>> +
>>
>> static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
>> ...
>>     if (nr_msrs > 2 && (msrs[1].guest & msrs[0].guest)) {
>>         msrs[2].guest = pmu->ds_area;
>>         if (nr_msrs > 3)
>>             msrs[3].guest = pmu->pebs_data_cfg;
>>     }
>>
>>    for (i = 0; i < nr_msrs; i++)
>> ...
>
> Yeah, that's exactly what I'm complaining about. Splitting the logic for
> determining the guest values is unnecessarily confusing, and as evidenced by the
> PEBS_ENABLE bug, potentially fragile. Perf should have full knowledge and
> control of what values are loaded for the guest. And, the above indexing magic
> is nigh impossible to follow and _super_ fragile.

Thanks for pointing this out.

>
> If we change .guest_get_msrs() to take a struct kvm_pmu pointer, then it can
> generate the full set of guest values by grabbing ds_area and pebs_data_cfg.
> Alternatively, .guest_get_msrs() could take the desired guest MSR values
> directly (ds_area and pebs_data_cfg), but kvm_pmu is vendor agnostic, so I don't
> see any reason to not just pass the pointer.

Hi Peter,

What do you think of us passing a "struct kvm_pmu" pointer (defined in
arch/x86/include/asm/kvm_host.h) to guest_get_msrs(int *nr) ?

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thx,likexu