linux-kernel - Re: [PATCH] KVM: X86: Enable IA32_MSIC_ENABLE MONITOR bit when exposing mwait/monitor

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Date:   Tue, 14 May 2019 14:13:50 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH] KVM: X86: Enable IA32_MSIC_ENABLE MONITOR bit when
 exposing mwait/monitor

On Mon, 13 May 2019 at 21:35, Radim Krčmář <rkrcmar@...hat.com> wrote:
>
> 2019-05-13 17:46+0800, Wanpeng Li:
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > MSR IA32_MSIC_ENABLE bit 18, according to SDM:
> >
> >  | When this bit is set to 0, the MONITOR feature flag is not set (CPUID.01H:ECX[bit 3] = 0).
> >  | This indicates that MONITOR/MWAIT are not supported.
> >  |
> >  | Software attempts to execute MONITOR/MWAIT will cause #UD when this bit is 0.
> >  |
> >  | When this bit is set to 1 (default), MONITOR/MWAIT are supported (CPUID.01H:ECX[bit 3] = 1).
> >
> > This bit should be set to 1, if BIOS enables MONITOR/MWAIT support on host and
> > we intend to expose mwait/monitor to the guest.
>
> The CPUID.01H:ECX[bit 3] ought to mirror the value of the MSR bit and
> as userspace has control of them both, I'd argue that it is userspace's
> job to configure both bits to match on the initial setup.
>
> Also, CPUID.01H:ECX[bit 3] is a better guard than kvm_mwait_in_guest().
> kvm_mwait_in_guest() affects the behavior of MONITOR/MWAIT, not its
> guest visibility.
> Some weird migration cases might want MONITOR in CPUID without
> kvm_mwait_in_guest() and the MSR should be correct there as well.
>
> Missing the MSR bit shouldn't be a big problem for guests, so I am in
> favor of fixing the userspace code.
>
> Thanks.
>
> (For extra correctness in KVM, we could implement toggling of the CPUID
>  bit based on guest writes to the MSR.)

Do it in v2 and a separate qemu patch, thanks for your review.

Regards,
Wanpeng Li