Re: [Qemu-devel] [PATCH 00/17] RFC: userfault v2

[zhanghailiang]

On 2014/11/21 1:38, Andrea Arcangeli wrote:
> Hi,
>
> On Thu, Nov 20, 2014 at 10:54:29AM +0800, zhanghailiang wrote:
> > Yes, you are right. This is what i really want, bypass all non-present faults
> > and only track strict wrprotect faults. ;)
> >
> > So, do you plan to support that in the userfault API?
>
> Yes I think it's good idea to support wrprotect/COW faults too.

Great! Then i can expect your patches. ;)

> I just wanted to understand if there was any other reason why you
> needed only wrprotect faults, because the non-present faults didn't
> look like a big performance concern if they triggered in addition to
> wrprotect faults, but it's certainly ok to optimize them away so it's
> fully optimal.

Er, you have got the answer, no special, it's only for optimality.

> All it takes to differentiate the behavior should be one more bit
> during registration so you can select non-present, wrprotect faults or
> both. postcopy live migration would select only non-present faults,
> postcopy live snapshot would select only wrprotect faults, anything
> like distributed shared memory supporting shared readonly access and
> exclusive write access, would select both flags.

It is really flexible in this way.

> I just sent an (unfortunately) longish but way more detailed email
> about live snapshotting with userfaultfd but I just wanted to give a
> shorter answer here too :).

Thanks for your explanation, and your patience. It is really useful,
now i know more details about why 'fork & dump live snapshot' scenario
is not acceptable. Thanks :)

> Thanks,
> Andrea
>
> .