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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Nov 18, 2010 at 03:12:56PM +0800, Xiao Guangrong wrote:
> On 11/17/2010 11:57 PM, Avi Kivity wrote:
> 
> >>>   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.
> > 
> 
> Maybe it's not a problem if guest can flush all tlbs after overwrite it?
> Marcelo, what's your comment about this?

It can, but there is no guarantee. Your patch is correct.