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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Apr 20, 2012 at 09:40:30PM -0300, Marcelo Tosatti wrote:
> 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 As soon as guest writes to gpte, information in bit is outdated.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. :(

OK, i now see you mentioned a similar case in the document, for
rmap_write_protect.

More importantly than the particular flush TLB case, the point is
every piece of code that reads and writes sptes must now be aware that
mmu_lock alone does not guarantee stability. Everything must be audited.

Where the bulk of the improvement comes from again? If there is little
or no mmu_lock contention (which we have no consistent data to be honest
in your testcase) is the bouncing off mmu_lock's cacheline that hurts?