Re: KVM: MMU: optimize set_spte for page sync

[Marcelo Tosatti <mtosatti@redhat.com>]

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.