linux-kernel - Re: [PATCH 2/2] KVM: x86: zero MSR_IA32_FEATURE

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Message-ID: <20160708125235.tz3oh5tycwhzydqo@hz-desktop>
Date:	Fri, 8 Jul 2016 20:52:35 +0800
From:	Haozhong Zhang <haozhong.zhang@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	rkrcmar@...hat.com
Subject: Re: [PATCH 2/2] KVM: x86: zero MSR_IA32_FEATURE_CONTROL on reset

On 07/08/16 14:01, Paolo Bonzini wrote:
> Reported-by: Laszlo Ersek <lersek@...hat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 388d9ffd7551..c01b0b3a06aa 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7494,6 +7494,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	if (!init_event) {
>  		kvm_pmu_reset(vcpu);
>  		vcpu->arch.smbase = 0x30000;
> +		vcpu->arch.msr_ia32_feature_control = 0;
>  	}
>  
>  	memset(vcpu->arch.regs, 0, sizeof(vcpu->arch.regs));
> -- 
> 1.8.3.1
> 

Actually I'm not sure whether zeroing MSR_IA32_FEATURE_CONTROL on
reset is the accurate behavior.

I know as Laszlo reported that Intel SDM says

  VMXON is also controlled by the IA32_FEATURE_CONTROL MSR (MSR
  address 3AH). This MSR is *cleared to zero* when a logical processor
  is reset

However, when the later section "ARCHITECTURAL MSRS" in Chapter
"MODEL-SPECIFIC REGISTERS (MSRS)" explains the Lock bit of
MSR_IA32_FEATURE_CONTROL in Table "IA-32 Architectural MSRs", it says

  ... once the Lock bit is set, the entire IA32_FEATURE_CONTROL
  contents are *preserved* across RESET when PWRGOOD is not
  deasserted.

This looks like a conflict. I'm asking my Intel colleagues for the
accurate behavior.

Thanks,
Haozhong