Re: [PATCH v2 2/6] KVM: MMU: don't drop spte if overwrite it fromW to RO

[Avi Kivity]

On 11/17/2010 05:42 PM, Marcelo Tosatti wrote:
> On Wed, Nov 17, 2010 at 12:10:50PM +0800, Xiao Guangrong wrote:
> > We just need flush tlb if overwrite a writable spte with a read-only
> > one.
> >
> > And we should move this operation to set_spte() for sync_page path
> >
> > Signed-off-by: Xiao Guangrong<xiaoguangrong@xxxxxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/mmu.c |   20 +++++++++-----------
> >   1 files changed, 9 insertions(+), 11 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index e008ae7..9bad960 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -1966,7 +1966,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
> >   		    gfn_t gfn, pfn_t pfn, bool speculative,
> >   		    bool can_unsync, bool reset_host_protection)
> >   {
> > -	u64 spte;
> > +	u64 spte, entry = *sptep;
> >   	int ret = 0;
> >
> >   	/*
> > @@ -2039,6 +2039,14 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
> >
> >   set_pte:
> >   	update_spte(sptep, spte);
> > +	/*
> > +	 * If we overwrite a writable spte with a read-only one we
> > +	 * should flush remote TLBs. Otherwise rmap_write_protect
> > +	 * will find a read-only spte, even though the writable spte
> > +	 * might be cached on a CPU's TLB.
> > +	 */
> > +	if (is_writable_pte(entry)&&  !is_writable_pte(*sptep))
> > +		kvm_flush_remote_tlbs(vcpu->kvm);
> There is no need to flush on sync_page path since the guest is
> responsible for it.

 If we don't, the next rmap_write_protect() will incorrectly decide 
that there's no need to flush tlbs.

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