linux-kernel - Re: [PATCH 3/5] KVM: vmx: don't vmx_segment_cache_clear twice in enter

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Date:	Mon, 18 Aug 2014 12:24:54 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>
CC:	Gleb Natapov <gleb@...nel.org>, hpa@...or.com, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] KVM: vmx: don't vmx_segment_cache_clear twice in
 enter_pmode

Il 18/08/2014 11:50, Wanpeng Li ha scritto:
> vmx_segment_cache_clear() will be called by vmx_set_segment() 
> which lead to vmx_segment_cache_clear() is called twice in 
> enter_pmode(). This patch remove the duplicate call site. 
> 
> Reviewed-by: Yang Zhang <yang.z.zhang@...el.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
> ---
>  arch/x86/kvm/vmx.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 2963303..c97c44c 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3183,8 +3183,6 @@ static void enter_pmode(struct kvm_vcpu *vcpu)
>  
>  	vmx->rmode.vm86_active = 0;
>  
> -	vmx_segment_cache_clear(vmx);
> -
>  	vmx_set_segment(vcpu, &vmx->rmode.segs[VCPU_SREG_TR], VCPU_SREG_TR);
>  
>  	flags = vmcs_readl(GUEST_RFLAGS);
> 

I think this is not useful and obscures the code a bit.

It would be a bigger improvement to see when fix_pmode_seg really needs
to call vmx_set_segment, and avoid the call.  Are you seeing enter_pmode
high in any profiles?

Paolo
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