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Date: Mon, 12 Apr 2010 11:34:47 +0300
From: Avi Kivity <avi@redhat.com>
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To: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
CC: Marcelo Tosatti <mtosatti@redhat.com>, KVM list <kvm@vger.kernel.org>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/6] KVM MMU: optimize for writing cr4
References: <4BC2D2E2.1030604@cn.fujitsu.com> <4BC2D3E0.8070602@cn.fujitsu.com>
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On 04/12/2010 11:03 AM, Xiao Guangrong wrote:
> Usually, OS changes CR4.PGE bit to flush all global page, under this
> case, no need reset mmu and just flush tlb
>
> Signed-off-by: Xiao Guangrong<xiaoguangrong@cn.fujitsu.com>
> ---
>   arch/x86/kvm/x86.c |    9 +++++++++
>   1 files changed, 9 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fd5c3d3..2aaa6fb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -463,6 +463,15 @@ void kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>   	unsigned long old_cr4 = kvm_read_cr4(vcpu);
>   	unsigned long pdptr_bits = X86_CR4_PGE | X86_CR4_PSE | X86_CR4_PAE;
>
> +	if (cr4 == old_cr4)
> +		return;
> +
> +	if ((cr4 ^ old_cr4) == X86_CR4_PGE) {
> +		kvm_mmu_sync_roots(vcpu);
> +		kvm_mmu_flush_tlb(vcpu);
> +		return;
> +	}
> +
>   	if (cr4&  CR4_RESERVED_BITS) {
>   		kvm_inject_gp(vcpu, 0);
>   		return;
>    

Later we have:

>         kvm_x86_ops->set_cr4(vcpu, cr4);
>         vcpu->arch.cr4 = cr4;
>         vcpu->arch.mmu.base_role.cr4_pge = (cr4 & X86_CR4_PGE) && 
> !tdp_enabled;

All of which depend on cr4.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.