Re: KVM: MMU: only write protect mappings at pagetable level

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Dec 22, 2010 at 03:01:19PM +0200, Avi Kivity wrote:
> On 12/22/2010 01:12 PM, Marcelo Tosatti wrote:
> >On Wed, Dec 22, 2010 at 01:07:23PM +0200, Avi Kivity wrote:
> >>  On 12/22/2010 01:01 PM, Marcelo Tosatti wrote:
> >>  >If a pagetable contains a writeable large spte, all of its sptes will be
> >>
> >>  non-writeable
> >>
> >>  >write protected, including non-leaf ones, leading to endless pagefaults.
> >>  >
> >>  >Do not write protect pages above PT_PAGE_TABLE_LEVEL, as the spte fault
> >>  >paths assume non-leaf sptes are writable.
> >>  >
> >>  >Signed-off-by: Marcelo Tosatti<mtosatti@redhat.com>
> >>  >
> >>  >diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> >>  >index c3853d5..c716ff8 100644
> >>  >--- a/arch/x86/kvm/mmu.c
> >>  >+++ b/arch/x86/kvm/mmu.c
> >>  >@@ -3442,6 +3442,9 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
> >>  >   		if (!test_bit(slot, sp->slot_bitmap))
> >>  >   			continue;
> >>  >
> >>  >+		if (sp->role.level != PT_PAGE_TABLE_LEVEL)
> >>  >+			continue;
> >>  >+
> >>  >   		pt = sp->spt;
> >>  >   		for (i = 0; i<   PT64_ENT_PER_PAGE; ++i)
> >>  >   			/* avoid RMW */
> >>
> >>  But what about large leaf sptes?  Don't we want to write protect, or
> >>  perhaps drop them?
> >>
> >>  I think write-protecting leaf sptes and ignoring nonleaf sptes should work.
> >
> >When dirty logging is enabled large sptes are nuked and creation of new
> >ones is not allowed. So i don't see the need?
> 
> Where does this nuking happen?
> 
> All I see is the call to kvm_mmu_slot_remove_write_access().

set_memory_region:

                /* destroy any largepage mappings for dirty tracking */
                if (old.npages)
                        flush_shadow = 1;