linux-kernel - Re: [PATCH v5 06/13] KVM: x86/vmx: Save/Restore host MSR_ARCH_LBR

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Message-ID: <20210713094713.GB13824@intel.com>
Date:   Tue, 13 Jul 2021 17:47:13 +0800
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, pbonzini@...hat.com,
        seanjc@...gle.com, vkuznets@...hat.com, wei.w.wang@...el.com,
        like.xu.linux@...il.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 06/13] KVM: x86/vmx: Save/Restore host
 MSR_ARCH_LBR_CTL state

On Mon, Jul 12, 2021 at 10:23:02AM -0700, Jim Mattson wrote:
> On Mon, Jul 12, 2021 at 2:36 AM Yang Weijiang <weijiang.yang@...el.com> wrote:
> >
> > On Fri, Jul 09, 2021 at 03:54:53PM -0700, Jim Mattson wrote:
> > > On Fri, Jul 9, 2021 at 2:51 AM Yang Weijiang <weijiang.yang@...el.com> 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 host
> > > architectural LBR MSRs have to be saved on vcpu_load and restored on
> > > vcpu_put. Shouldn't  kvm_load_guest_fpu() and kvm_put_guest_fpu() do
> > > that via the calls to kvm_save_current_fpu(vcpu->arch.user_fpu) and
> > > restore_fpregs_from_fpstate(&vcpu->arch.user_fpu->state)?
> > I looked back on the discussion thread:
> > https://patchwork.kernel.org/project/kvm/patch/20210303135756.1546253-8-like.xu@linux.intel.com/
> > not sure why this code is added, but IMO, although fpu save/restore in outer loop
> > covers this LBR MSR, but the operation points are far away from vm-entry/exit
> > point, i.e., the guest MSR setting could leak to host side for a signicant
> > long of time, it may cause host side profiling accuracy. if we save/restore it
> > manually, it'll mitigate the issue signifcantly.
> 
> I'll be interested to see how you distinguish the intermingled branch
> streams, if you allow the host to record LBRs while the LBR MSRs
> contain guest values!
I'll check if an inner simplified xsave/restore to guest/host LBR MSRs is meaningful,
the worst case is to drop this patch since it's not correct to only enable host lbr ctl
while still leaves guest LBR data in the MSRs. Thanks for the reminder!