Re: [PATCH 2/2] kvm: change the dirty page tracking to work with dirty bity

[Izik Eidus <ieidus@redhat.com>]

Marcelo Tosatti wrote:
>
>
> What i'm saying is with shadow and NPT (i believe) you can mark a spte
> writable but not dirty, which gives you the ability to know whether
> certain pages have been dirtied.
>   

Isnt this what this patch is doing?

> Should be ok todo that with shadow, as long as the gpte is already
> dirty, right? So:
>
> - guest create writable pte
> - guest write fault on that pte
> - vmexit
> - write dirty bit to gpte
> - create writable spte
> -> 
>
> from this point on, you can create a writable spte which is not dirtied, 
> and use the dirty bit to know whether a page has been written to?
>
> And unless i'm mistaken NPT gives you that for free since the hw updates
> the spte with accessed/dirty bits.
>   

Yes

> Maybe this is a stupid idea. 
>   

I dont sure if i am missing it, or if this what i tried to do with this 
patch.

>   
>>> * EPT violations do no transfer fault information to the page fault
>>> handler, but its available (there's a vm-exit field).
>>>   
>>>       
>> EPT in this patch, run as before, (using page faults with  
>> mark_page_dirty() inside set_spte())
>>     
>
> Right, i was talking about gup(write=0) when guest fault is read-only to
> relief KSM of wrprotecting host ptes, but that is quite different.
>
>   
>>>   
>>>       
>>>>  	r = -ENOMEM;
>>>>  @@ -1279,6 +1282,7 @@ int kvm_get_dirty_log(struct kvm *kvm,
>>>>  	if (!memslot->dirty_bitmap)
>>>>  		goto out;
>>>>  +	kvm_arch_get_dirty_log(kvm, memslot);
>>>>  	n = ALIGN(memslot->npages, BITS_PER_LONG) / 8;
>>>>   	for (i = 0; !any && i < n/sizeof(long); ++i)
>>>> -- 
>>>> 1.5.6.5
>>>>
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>>>>     
>>>>         
>> Thanks :).
>>