Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spteand mmu notifier to help KSM dirty bit tracking

From: Avi Kivity
Date: Wed Jun 22 2011 - 07:28:27 EST

On 06/22/2011 02:24 PM, Avi Kivity wrote:
On 06/22/2011 02:19 PM, Izik Eidus wrote:
On 6/22/2011 2:10 PM, Avi Kivity wrote:
On 06/22/2011 02:05 PM, Izik Eidus wrote:
+ spte = rmap_next(kvm, rmapp, NULL);
+ while (spte) {
+ int _dirty;
+ u64 _spte = *spte;
+ BUG_ON(!(_spte& PT_PRESENT_MASK));
+ _dirty = _spte& PT_DIRTY_MASK;
+ if (_dirty) {
+ dirty = 1;
+ clear_bit(PT_DIRTY_SHIFT, (unsigned long *)spte);
+ }

Racy. Also, needs a tlb flush eventually.

Hi, one of the issues is that the whole point of this patch is not do tlb flush eventually,
But I see your point, because other users will not expect such behavior, so maybe there is need into a parameter
flush_tlb=?, or add another mmu notifier call?

If you don't flush the tlb, a subsequent write will not see that spte.d is clear and the write will happen. So you'll see the page as clean even though it's dirty. That's not acceptable.

Yes, but this is exactly what we want from this use case:
Right now ksm calculate the page hash to see if it was changed, the idea behind this patch is to use the dirty bit instead,
however the guest might not really like the fact that we will flush its tlb over and over again, specially in periodically scan like ksm does.

I see.

Actually, this is dangerous. If we use the dirty bit for other things, we will get data corruption.

For example we might want to map clean host pages as writeable-clean in the spte on a read fault so that we don't get a page fault when they get eventually written.

error compiling committee.c: too many arguments to function

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