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

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Date:   Mon, 12 Jul 2021 17:50:34 +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 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.