Re: [RFC PATCH 0/2] KVM: VMX: Fix VM entry failure on PT_MODE_HOST_GUEST while host is using PT

Liang, Kan
2022-09-19T14:41:38Z

On 2022-09-19 9:46 a.m., Wang, Wei W wrote:
> On Friday, September 16, 2022 9:27 PM, Liang, Kan wrote:
>>> Did you mean to handle the PT event in the proposed driver API? Event
>>> status is just one of the things. There are other things if we want to
>>> make it complete for this, e.g. event->oncpu = -1, and eventually seems we will
>> re-implement perf_event_disable_*.
>>>
>>
>> As my understand, perf always check the status first. If it's a stopped or
>> inactivated event, I don't think event->oncpu will be touched. That's why I think
>> the proposed driver API should be acceptable.
>
> That's the implementation thing. We need to make it architecturally clean though.
>
>>
>>> Btw, Xiaoyao has made it work with perf_event_disable_local, and don’t have
>> that many changes.
>>> If necessary, we can post the 2nd version out to double check.
>>>
>>
>> I'm not worry about which ways (either perf_event_disable_local() or the
>> proposed PT driver API) are chosen to stop the PT. If the existing perf_event
>> interfaces can meet your requirement, that's perfect.
>>
>> My real concern is the pt_save_msr()/pt_load_msr(). I don't think it's a job for
>> KVM. See atomic_switch_perf_msrs(). It is the perf core driver rather than KVM
>> that tells which MSRs should be saved/restored in VMCS.
>> We should do the same thing for PT. (Actually, I think we already encounter
>> issues with the current KVM-dominated method. KVM saves/restores
>> unnecessary MSRs. Right?)
>>
>
> Right. It's on my plan to improve the current PT virtualization, and
> planed to be the next step after this fix. The general rule is the same: make KVM a user
> of perf, that is, we leave those save/restore work to be completely done by the
> perf (driver) side, so we will eventually remove the KVM side pt_save/load_msr.
> To be more precise, it will work as below:
> - we will create a guest event, like what we did for lbr virtualization

Another fake event? We have to specially handle it in the perf code. I
don't think it's a clean way for perf.

> - on VMEnter:
> -- perf_disable_event_local(host_event);
> -- perf_enable_event_local(guest_event);
> - on VMExit:
> -- perf_disable_event_local(guest_event);
> -- perf_enable_event_local(host_event);

Why we cannot use the same way as the perf core driver to switch the
MSRs in the VMCS?

You just need one generic function, perf_guest_get_msrs(), for both PT
and core driver. If you have to disable PT explicitly before VMCS, I
think you can do it in the PT specific perf_guest_get_msrs().

Anyway, that's an improvement for the current code. I don't have a
problem, if you prefer to separate the fix patch and improvement patch.

Thanks,
Kan
>
>> To do so, I think there may be two ways.
>> - Since MSRs have to be switched for both PT and core drivers, it sounds
>> reasonable to provide a new generic interface in the perf_event. The new
>> interface is to tell KVM which MSRs should be saved/restored. Then KVM can
>> decide to save/restore via VMCS or direct MSR access. I suspect this way
>> requires big change, but it will benefit all the drivers which have similar
>> requirements.
>> - The proposed driver API. The MSRs are saved/restored in the PT driver.
>
> As shown above, no need for those. We can completely reuse the
> perf side save/restore.
>
> Thanks,
> Wei