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From:   Jim Mattson <jmattson@google.com>
Date:   Mon, 12 Jul 2021 10:45:10 -0700
Message-ID: <CALMp9eSWDmWerj5CaxRyMiNqnBf1akHHaWV2Cfq_66Xjg-0MEw@mail.gmail.com>
Subject: Re: [PATCH v5 06/13] KVM: x86/vmx: Save/Restore host MSR_ARCH_LBR_CTL state
To:     Like Xu <like.xu.linux@gmail.com>
Cc:     Yang Weijiang <weijiang.yang@intel.com>, pbonzini@redhat.com,
        seanjc@google.com, vkuznets@redhat.com, wei.w.wang@intel.com,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        "kan.liang@linux.intel.com" <kan.liang@linux.intel.com>
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On Mon, Jul 12, 2021 at 10:20 AM Jim Mattson <jmattson@google.com> wrote:
>
> On Mon, Jul 12, 2021 at 3:19 AM Like Xu <like.xu.linux@gmail.com> wrote:
> >
> > On 12/7/2021 5:53 pm, Yang Weijiang wrote:
> > > On Fri, Jul 09, 2021 at 04:41:30PM -0700, Jim Mattson wrote:
> > >> On Fri, Jul 9, 2021 at 3:54 PM Jim Mattson <jmattson@google.com> wro=
te:
> > >>>
> > >>> On Fri, Jul 9, 2021 at 2:51 AM Yang Weijiang <weijiang.yang@intel.c=
om> wrote:
> > >>>>
> > >>>> If host is using MSR_ARCH_LBR_CTL then save it before vm-entry
> > >>>> and reload it after vm-exit.
> > >>>
> > >>> I don't see anything being done here "before VM-entry" or "after
> > >>> VM-exit." This code seems to be invoked on vcpu_load and vcpu_put.
> > >>>
> > >>> In any case, I don't see why this one MSR is special. It seems that=
 if
> > >>> the host is using the architectural LBR MSRs, then *all* of the hos=
t
> > >>> architectural LBR MSRs have to be saved on vcpu_load and restored o=
n
> > >>> vcpu_put. Shouldn't  kvm_load_guest_fpu() and kvm_put_guest_fpu() d=
o
> > >>> that via the calls to kvm_save_current_fpu(vcpu->arch.user_fpu) and
> > >>> restore_fpregs_from_fpstate(&vcpu->arch.user_fpu->state)?
> > >>
> > >> It does seem like there is something special about IA32_LBR_DEPTH, t=
hough...
> > >>
> > >> Section 7.3.1 of the Intel=C2=AE Architecture Instruction Set Extens=
ions
> > >> and Future Features Programming Reference
> > >> says, "IA32_LBR_DEPTH is saved by XSAVES, but it is not written by
> > >> XRSTORS in any circumstance." It seems like that would require some
> > >> special handling if the host depth and the guest depth do not match.
> > > In our vPMU design, guest depth is alway kept the same as that of hos=
t,
> > > so this won't be a problem. But I'll double check the code again, tha=
nks!
> >
> > KVM only exposes the host's depth value to the user space
> > so the guest can only use the same depth as the host.
>
> The allowed depth supplied by KVM_GET_SUPPORTED_CPUID isn't enforced,
> though, is it?

Also, doesn't this end up being a major constraint on future
platforms? Every host that this vCPU will ever run on will have to use
the same LBR depth as the host on which it was started.