linux-kernel - Re: [PATCH 04/12] KVM: MMU: log dirty page after marking spte writable

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Message-ID: <51F7BF00.5060703@redhat.com>
Date:	Tue, 30 Jul 2013 15:26:24 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC:	gleb@...hat.com, avi.kivity@...il.com, mtosatti@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 04/12] KVM: MMU: log dirty page after marking spte writable

Il 30/07/2013 15:02, Xiao Guangrong ha scritto:
> Make sure we can see the writable spte before the dirt bitmap is visible
> 
> We do this is for kvm_vm_ioctl_get_dirty_log() write-protects the spte based
> on the dirty bitmap, we should ensure the writable spte can be found in rmap
> before the dirty bitmap is visible. Otherwise, we cleared the dirty bitmap and
> failed to write-protect the page
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
>  arch/x86/kvm/mmu.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 35d4b50..0fe56ad 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2486,12 +2486,12 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>  		}
>  	}
>  
> -	if (pte_access & ACC_WRITE_MASK)
> -		mark_page_dirty(vcpu->kvm, gfn);
> -
>  set_pte:
>  	if (mmu_spte_update(sptep, spte))
>  		kvm_flush_remote_tlbs(vcpu->kvm);
> +
> +	if (pte_access & ACC_WRITE_MASK)
> +		mark_page_dirty(vcpu->kvm, gfn);
>  done:
>  	return ret;
>  }
> 

What about this comment above:

                /*
                 * Optimization: for pte sync, if spte was writable the hash
                 * lookup is unnecessary (and expensive). Write protection
                 * is responsibility of mmu_get_page / kvm_sync_page.
                 * Same reasoning can be applied to dirty page accounting.
                 */
                if (!can_unsync && is_writable_pte(*sptep))
                        goto set_pte;

                if (mmu_need_write_protect(vcpu, gfn, can_unsync)) {


?

Should it be changed to

                if (!can_unsync && is_writable_pte(*sptep))
                        pte_access &= ~ACC_WRITE_MASK; /* do not mark dirty */

                else if (mmu_need_write_protect(vcpu, gfn, can_unsync)) {

?

Thanks,

Paolo
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