Re: [Xen-devel] [PATCH 1/2] x86/mm: remove arch-specific ptep_get_and_clear() function

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 15/06/2012 10:41, "David Vrabel" <david.vrabel@xxxxxxxxxx> wrote:

> This reasoning is probably not correct.  When a dirty bit must be
> updated in a PTE the processor does a pagetable walk (possibly using any
> cached page table structures).  The AMD APM section 5.4.2 states:
> 
> "The processor never sets the Accessed bit or the Dirty bit for a not
> present page (P = 0)."
> 
> and
> 
> "If PTE[D] is cleared to 0, software can rely on the fact that the page
> has not been written."

Writing of dirty and accessed bits is done as part of the page-table walk on
TLB fill. A/D bits never have writeback caching semantics. It wouldn't be
safe: e.g., on unmap, TLB flushes happen after ptes have been cleared (to
avoid TLB-fill races), but that would mean that A/D updates could be lost
even on non-explicit unmaps (e.g., page out) which is obviously bad.

> Thus this patch would /introduce/ a race where a dirty bit set would be
> lost (rather than extending the window where this would happen).
> 
> However (and this is a weaker argument), no sensible userspace
> application should be accessing pages that are being unmapped or
> remapped (since it is unpredictable whether they will fault) so perhaps
> this additional unpredictable behaviour is acceptable?

If there's a big win to be had through batching, we're better off devising a
hypercall method for capturing the atomic rmw operation as it stands, rather
than subtly messing with semantics.

 -- Keir



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