On 17/12/19 13:16, Christophe de Dinechin wrote: > > >> On 14 Dec 2019, at 08:57, Paolo Bonzini <pbonzini@xxxxxxxxxx> 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) Yes, that was the idea but Peter decided to scrap it. :) Paolo >> >> 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. >>> >> >
