From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: KVM: MMU: only write protect mappings at pagetable level
Date: Wed, 22 Dec 2010 09:12:59 -0200
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References: <20101222110157.GA4266@amt.cnet>
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To: Avi Kivity <avi@redhat.com>
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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?