Re: [PATCH v1 2/3] io: Add zerocopy and errqueue

[Peter Xu]

On Wed, Sep 08, 2021 at 10:57:06PM +0100, Daniel P. Berrangé wrote:
> We think we're probably ok with migration as we are going to rely on the
> face that we eventually pause the guest to stop page changes during the
> final switchover. None the less I really strongly dislike the idea of
> not honouring the kernel API contract, despite the potential performance
> benefits it brings.

Yes understandable, and it does looks tricky. But it's guest page and it's just
by nature how it works to me (sending page happening in parallel with page
being modified).

I think the MSG_ZEROCOPY doc page mentioned that and it's userspace program's
own choice if that happens. So even if it's not by design and not suggested, it
seems not forbidden either.

We can wr-protect the page (using things like userfaultfd-wp) during sending
and unprotect them when it's done with a qio callback, that'll guarantee the
buffer not changing during sending, however we gain nothing besides the "api
cleaness" then..

Thanks,

-- 
Peter Xu