linux-kernel - Re: [PATCH v2] KVM: x86: Reset RFLAGS state following processor init/reset

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Date:	Tue, 3 Nov 2015 12:54:56 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <wanpeng.li@...mail.com>
Cc:	Nadav Amit <nadav.amit@...il.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86: Reset RFLAGS state following processor
 init/reset

On 03/11/2015 12:40, Wanpeng Li wrote:
> Reference SDM Volume 1 3.4.3:
> 
> Following initialization of the processor (either by asserting the 
> RESET pin or the INIT pin), the state of the EFLAGS register is 
> 00000002H.
> 
> However, the eflags fixed bit is not set and other bits are also not 
> cleared during the init/reset in kvm.
> 
> This patch reset eflags register to 00000002H following initialization 
> of the processor.
> 
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> v1 -> v2:
>  * use vmcs_writel
> 
>  arch/x86/kvm/vmx.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index b680c2e..1a95ef7 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -4935,6 +4935,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	vmx_set_efer(vcpu, 0);
>  	vmx_fpu_activate(vcpu);
>  	update_exception_bitmap(vcpu);
> +	vmcs_writel(GUEST_RFLAGS, X86_EFLAGS_FIXED);
>  
>  	vpid_sync_context(vmx->vpid);
>  }
> 

No, this is doing exactly the same thing that is already done elsewhere
in vmx_vcpu_reset (which Nadav pointed out to you).  So it's not just a
pointless addition with no effect at all; it's wrong, because it
introduces duplication.

Please answer this question: is there a bug or not?

If yes, then using kvm_set_rflags as in v1 is the right thing.  However,
you have to remove the _existing_ vmcs_writel call in vmx_vcpu_reset.
Also, if there is a bug you have to explain it in the commit message and
provide a testcase.  By the way, I am still waiting for the VPID test cases.

If no, then this is a cleanup, we can still do the change but you have
to explain this in the commit message.

Paolo
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