Re: [patch 09/10] KVM: MMU: out of sync shadow core v2

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Sep 22, 2008 at 11:41:14PM +0300, Avi Kivity wrote:
> Marcelo Tosatti wrote:
>>>> +	while (parent->unsync_children) {
>>>> +		for (i = 0; i < PT64_ENT_PER_PAGE; ++i) {
>>>> +			u64 ent = sp->spt[i];
>>>> +
>>>> +			if (is_shadow_present_pte(ent)) {
>>>> +				struct kvm_mmu_page *child;
>>>> +				child = page_header(ent & PT64_BASE_ADDR_MASK);
>>>>       
>>> What does this do?
>>>     
>>
>> Walks all children of given page with no efficiency. Its replaced later
>> by the bitmap version.
>>
>>   
>
> I don't understand how the variables sp, child, and parent interact. You  
> either need recursion or an explicit stack?

It restarts at parent level whenever finishing any children:

+               if (i == PT64_ENT_PER_PAGE) {
+                       sp->unsync_children = 0;
+                       sp = parent;
+               }

No efficiency.

>> Yes. The next element for_each_entry_safe saved could have been zapped.
>>
>>   
>
> Ouch. Ouch.
>
> I hate doing this. Can see no alternative though.

Me neither.

>>>> +	/* don't unsync if pagetable is shadowed with multiple roles */
>>>> +	hlist_for_each_entry_safe(s, node, n, bucket, hash_link) {
>>>> +		if (s->gfn != sp->gfn || s->role.metaphysical)
>>>> +			continue;
>>>> +		if (s->role.word != sp->role.word)
>>>> +			return 1;
>>>> +	}
>>>>         
>>> This will happen for nonpae paging.  But why not allow it?  Zap all   
>>> unsynced pages on mode switch.
>>>
>>> Oh, if a page is both a page directory and page table, yes.      
>>
>> Yes. 
>>
>>   
>>> So to allow nonpae oos, check the level instead.
>>>     
>>
>> Windows 2008 64-bit has all sorts of sharing a pagetable at multiple
>> levels too.
>>
>>   
>
> We still want to allow oos for the two quadrants of a nonpae shadow page.

Sure, can be an optimization step later?