On Sun, Nov 23, 2008 at 12:36:29PM +0200, Avi Kivity wrote:
> Marcelo Tosatti wrote:
>
>> The cost of hash table and memslot lookups are quite significant if the
>> workload is pagetable write intensive resulting in increased mmu_lock
>> contention.
>>
>> @@ -1593,7 +1593,16 @@ static int set_spte(struct kvm_vcpu *vcp
>> spte |= PT_WRITABLE_MASK;
>> - if (mmu_need_write_protect(vcpu, gfn, can_unsync)) {
>> + /*
>> + * Optimization: for pte sync, if spte was writable the hash
>> + * lookup is unnecessary (and expensive). Write protection
>> + * is responsibility of mmu_get_page / kvm_sync_page.
>> + * Same reasoning can be applied to dirty page accounting.
>> + */
>> + if (sync_page && is_writeble_pte(*shadow_pte))
>> + goto set_pte;
>>
>
> What if *shadow_pte points at a different page? Is that possible?
To a different gfn? Then sync_page will have nuked the spte:
if (gpte_to_gfn(gpte) != gfn || !is_present_pte(gpte) ||
!(gpte & PT_ACCESSED_MASK)) {
u64 nonpresent;
..
set_shadow_pte(&sp->spt[i], nonpresent);
}
Otherwise:
/*
* Using the cached information from sp->gfns is safe because:
* - The spte has a reference to the struct page, so the pfn for a given
* gfn can't change unless all sptes pointing to it are nuked first.
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