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

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Message-ID: <CALMp9eQ+9czB0ayBFR3-nW-ynKuH0v9uHAGeV4wgkXYJMSs1=w@mail.gmail.com>
Date:   Fri, 9 Jul 2021 16:41:30 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     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 9, 2021 at 3:54 PM Jim Mattson <jmattson@...gle.com> 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)?

It does seem like there is something special about IA32_LBR_DEPTH, though...

Section 7.3.1 of the Intel® Architecture Instruction Set Extensions
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.