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Subject: Re: [PATCH RESEND 2/4] KVM: X86: Emulate MSR_IA32_MISC_ENABLE MWAIT
 bit
To:     Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Liran Alon <liran.alon@oracle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
References: <1558082990-7822-1-git-send-email-wanpengli@tencent.com>
 <1558082990-7822-2-git-send-email-wanpengli@tencent.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <e96eecd6-7095-58b3-32a7-2cfde2f2ebcc@redhat.com>
Date:   Mon, 20 May 2019 12:34:56 +0200
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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