Re: [PATCH 06/12] KVM: MMU: flush tlb if the spte can be locklessly modified

[Gleb Natapov <gleb@redhat.com>]

On Tue, Jul 30, 2013 at 09:02:04PM +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@linux.vnet.ibm.com>
> ---
>  arch/x86/kvm/mmu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 58283bf..5a40564 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -600,7 +600,8 @@ static bool mmu_spte_update(u64 *sptep, u64 new_spte)
>  	 * we always atomicly update it, see the comments in
>  	 * spte_has_volatile_bits().
>  	 */
> -	if (is_writable_pte(old_spte) && !is_writable_pte(new_spte))
> +	if (spte_is_locklessly_modifiable(old_spte) &&
> +	      !is_writable_pte(new_spte))
>  		ret = true;
This will needlessly flush tlbs when dirty login is not in use (common
case) and old spte is non writable. Can you estimate how serious the
performance hit is?

>  
>  	if (!shadow_accessed_mask)
> -- 
> 1.8.1.4

--
			Gleb.