linux-kernel - RE: [RFC PATCH 0/2] KVM: VMX: Fix VM entry failure on PT_MODE_HOST

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Date:   Mon, 19 Sep 2022 15:22:58 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        "Li, Xiaoyao" <xiaoyao.li@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Arnaldo Carvalho de Melo" <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Liang, Kan" <kan.liang@...el.com>,
        "Kleen, Andi" <andi.kleen@...el.com>
CC:     "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [RFC PATCH 0/2] KVM: VMX: Fix VM entry failure on
 PT_MODE_HOST_GUEST while host is using PT

On Monday, September 19, 2022 10:41 PM, Liang, Kan wrote:
> Another fake event? We have to specially handle it in the perf code. I don't
> think it's a clean way for perf.

We can check the patch later. I think it should be clean, like the LBR side.

> 
> > - 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?

The current MSR switching list from VMCS isn’t fast,
should be the last resort when really necessary.

> 
> 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().

The disable is done via " Clear IA32_RTIT_CTL" VMExit control.
It can ensure PT disabled in time on VMExit, so no need to go through perf_guest_get_msrs.

Thanks,
Wei