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

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Date:   Mon, 20 May 2019 12:34:56 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     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 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
behind KVM_CAP_DISABLE_QUIRKS.  (Also, what is the reason for this
change in general besides making behavior closer to real hardware?)

Thanks,

Paolo