Re: [PATCH v3 5/9] KVM: MMU: introduce SPTE_WRITE_PROTECT bit

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Apr 20, 2012 at 06:52:11PM -0300, Marcelo Tosatti wrote:
> On Fri, Apr 20, 2012 at 04:19:17PM +0800, Xiao Guangrong wrote:
> > If this bit is set, it means the W bit of the spte is cleared due
> > to shadow page table protection
> > 
> > Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> > ---
> >  arch/x86/kvm/mmu.c |   56 ++++++++++++++++++++++++++++++++++-----------------
> >  1 files changed, 37 insertions(+), 19 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index dd984b6..eb02fc4 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -147,6 +147,7 @@ module_param(dbg, bool, 0644);
> > 
> >  #define SPTE_HOST_WRITEABLE	(1ULL << PT_FIRST_AVAIL_BITS_SHIFT)
> >  #define SPTE_ALLOW_WRITE	(1ULL << (PT_FIRST_AVAIL_BITS_SHIFT + 1))
> > +#define SPTE_WRITE_PROTECT	(1ULL << (PT_FIRST_AVAIL_BITS_SHIFT + 2))
> > 
> >  #define SHADOW_PT_INDEX(addr, level) PT64_INDEX(addr, level)
> > 
> > @@ -1042,36 +1043,51 @@ static void drop_spte(struct kvm *kvm, u64 *sptep)
> >  		rmap_remove(kvm, sptep);
> >  }
> > 
> > +static bool spte_wp_by_dirty_log(u64 spte)
> > +{
> > +	WARN_ON(is_writable_pte(spte));
> > +
> > +	return (spte & SPTE_ALLOW_WRITE) && !(spte & SPTE_WRITE_PROTECT);
> > +}
> 
> Is the information accurate? Say:
> 
> - dirty log write protect, set SPTE_ALLOW_WRITE, clear WRITABLE.
> - shadow gfn, rmap_write_protect finds page not WRITABLE.
> - spte points to shadow gfn, but SPTE_WRITE_PROTECT is not set.
> 
> BTW,
> 
> "introduce SPTE_ALLOW_WRITE bit
> 
> This bit indicates whether the spte is allow to be writable that
> means the gpte of this spte is writable and the pfn pointed by
> this spte is writable on host"
> 
> Other than the fact that each bit should have one meaning, how
> can this bit be accurate without write protection of the gpte?
> 
> As soon as guest writes to gpte, information in bit is outdated.

Ok, i found one example where mmu_lock was expecting sptes not 
to change:


VCPU0				VCPU1

- read-only gpte
- read-only spte
- write fault
- spte = *sptep
				guest write to gpte, set writable bit
				spte writable
				parent page unsync
				guest write to gpte writable bit clear
				guest invlpg updates spte to RO
				sync_page
				enter set_spte from sync_page
- cmpxchg(spte) is now writable
[window where another vcpu can
cache spte with writable bit
set]

				if (is_writable_pte(entry) && !is_writable_pte(*sptep))
					kvm_flush_remote_tlbs(vcpu->kvm);

The flush is not executed because spte was read-only (which is 
a correct assumption as long as sptes updates are protected
by mmu_lock).

So this is an example of implicit assumptions which break if you update
spte without mmu_lock. Certainly there are more cases. :(