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

[Wang, Wei W]

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
- 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);

> 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