linux-kernel - Re: [PATCH RESEND 2/4] KVM: X86: Emulate MSR_IA32_MISC

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Message-ID: <CANRm+Cze1YGtsXibqmRvL=XNHNETH3ZcpH4HEy-7qwE4qPnA9Q@mail.gmail.com>
Date:   Mon, 20 May 2019 19:39:50 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Liran Alon <liran.alon@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH RESEND 2/4] KVM: X86: Emulate MSR_IA32_MISC_ENABLE MWAIT bit

On Mon, 20 May 2019 at 18:34, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 17/05/19 10:49, Wanpeng Li wrote:
> > 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).
> >
> > The CPUID.01H:ECX[bit 3] ought to mirror the value of the MSR bit,
> > 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.
> >
> > This patch implements toggling of the CPUID bit based on guest writes
> > to the MSR.
>
> Won't this disable mwait after migration, unless IA32_MISC_ENABLE is set
> correctly by firmware or userspace?  I think you need to hide this

Agreed.

> behind KVM_CAP_DISABLE_QUIRKS.  (Also, what is the reason for this
> change in general besides making behavior closer to real hardware?)

Just making behavior closer to real hardware. :)

Regards,
Wanpeng Li