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 10:20:04 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, pbonzini@...hat.com,
        seanjc@...gle.com, vkuznets@...hat.com, wei.w.wang@...el.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        "kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>
Subject: Re: [PATCH v5 06/13] KVM: x86/vmx: Save/Restore host MSR_ARCH_LBR_CTL state

On Mon, Jul 12, 2021 at 3:19 AM Like Xu <like.xu.linux@...il.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@...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.
> > In our vPMU design, guest depth is alway kept the same as that of host,
> > so this won't be a problem. But I'll double check the code again, thanks!
>
> 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?