On Wednesday 22 June 2011 19:28:08 Avi Kivity wrote:

On 06/22/2011 02:24 PM, Avi Kivity wrote:

quoted

On 06/22/2011 02:19 PM, Izik Eidus wrote:

quoted

On 6/22/2011 2:10 PM, Avi Kivity wrote:

quoted

On 06/22/2011 02:05 PM, Izik Eidus wrote:

quoted

quoted

quoted

+    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.

Yeah,yeah, I actually clarified in a reply letter to Chris about his similar
concern that we are currently the _only_ user. :)
We can add the flushing when someone else should rely on this bit.

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.

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