linux-kernel - Re: [PATCH v3 03/15] KVM: MMU: flush tlb if the spte can be locklessly modified

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Date:	Tue, 12 Nov 2013 22:10:30 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	gleb@...hat.com, avi.kivity@...il.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 03/15] KVM: MMU: flush tlb if the spte can be
 locklessly modified

On Wed, Oct 23, 2013 at 09:29:21PM +0800, Xiao Guangrong wrote:
> Relax the tlb flush condition since we will write-protect the spte out of mmu
> lock. Note lockless write-protection only marks the writable spte to readonly
> and the spte can be writable only if both SPTE_HOST_WRITEABLE and
> SPTE_MMU_WRITEABLE are set (that are tested by spte_is_locklessly_modifiable)
> 
> This patch is used to avoid this kind of race:
> 
>       VCPU 0                         VCPU 1
> lockless wirte protection:
>       set spte.w = 0
>                                  lock mmu-lock
> 
>                                  write protection the spte to sync shadow page,
>                                  see spte.w = 0, then without flush tlb
> 
> 				 unlock mmu-lock
> 
>                                  !!! At this point, the shadow page can still be
>                                      writable due to the corrupt tlb entry
>      Flush all TLB
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>

Reviewed-by: Marcelo Tosatti <mtosatti@...hat.com>

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