Re: [Minios-devel] [PATCH MINI-OS] xenbus: notify the other end when necessary

[Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>]

Wei Liu, le Mon 26 Oct 2015 12:30:28 +0000, a écrit :
> On Mon, Oct 26, 2015 at 01:21:51PM +0100, Samuel Thibault wrote:
> > Wei Liu, le Mon 26 Oct 2015 12:14:43 +0000, a écrit :
> > > In my patch, mini-os notifies remote whenever it consumes a message,
> > > which I think it's slightly better because backend can start putting
> > > things in the ring as mini-os processes them.
> > 
> > That makes more notifications, but that can lead to more pipelining
> > indeed.  That's what the Linux driver does, so let's do the same.
> > 
> > Also, I'm realizing: aren't we missing a full memory barrier between
> > the memcpy_from_ring call and xenstore_buf->rsp_cons += ? (in the two
> > places) We need to make sure to have finished copying from the ring
> > before writing the new rsp_cons.
> > 
> 
> You're right.
> 
> I think we should just turn that wmb() into two mb()s and place them
> before xenstore_buf->rsp_cons +=.

We *also* need some barrier between rsp_cons += and the notification,
otherwise the notified domain may miss the rsp_cons update and thus
believe it was a spurious notification.

Samuel

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