Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

Christophe de Dinechin
2019-12-17T12:16:37Z

> On 14 Dec 2019, at 08:57, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 13/12/19 21:23, Peter Xu wrote:
>>> What is the benefit of using u16 for that? That means with 4K pages, you
>>> can share at most 256M of dirty memory each time? That seems low to me,
>>> especially since it's sufficient to touch one byte in a page to dirty it.
>>>
>>> Actually, this is not consistent with the definition in the code ;-)
>>> So I'll assume it's actually u32.
>> Yes it's u32 now. Actually I believe at least Paolo would prefer u16
>> more. :)
>
> It has to be u16, because it overlaps the padding of the first entry.

Wow, now that’s subtle.

That definitely needs a union with the padding to make this explicit.

(My guess is you do that to page-align the whole thing and avoid adding a
page just for the counters)

>
> Paolo
>
>> I think even u16 would be mostly enough (if you see, the maximum
>> allowed value currently is 64K entries only, not a big one). Again,
>> the thing is that the userspace should be collecting the dirty bits,
>> so the ring shouldn't reach full easily. Even if it does, we should
>> probably let it stop for a while as explained above. It'll be
>> inefficient only if we set it to a too-small value, imho.
>>
>