> 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) > > 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. >> >
