Re: [PATCH v3 11/11] KVM: MMU: improve write flooding detected

Avi Kivity
2011-09-13T11:07:13Z

On 08/30/2011 05:38 AM, Xiao Guangrong wrote:
> Detecting write-flooding does not work well, when we handle page written, if
> the last speculative spte is not accessed, we treat the page is
> write-flooding, however, we can speculative spte on many path, such as pte
> prefetch, page synced, that means the last speculative spte may be not point
> to the written page and the written page can be accessed via other sptes, so
> depends on the Accessed bit of the last speculative spte is not enough
>
> Instead of detected page accessed, we can detect whether the spte is accessed
> after it is written, if the spte is not accessed but it is written frequently,
> we treat is not a page table or it not used for a long time
>
>

The spte may not be accessed, but other sptes in the same page can be
accessed. An example is the fixmap area for kmap_atomic(), there will
be a lot of pte writes but other sptes will be accessed without going
through soft-mmu at all.

I think you have to read the parent_ptes->spte.accessed bits to be sure.

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