Re: [RFC PATCH 13/27] vhost: Send buffers to device

From: Stefan Hajnoczi <stefanha@redhat.com>
To: Eugenio Perez Martin <eperezma@redhat.com>
Cc: kvm list <kvm@vger.kernel.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Stefan Hajnoczi <stefanha@gmail.com>,
	Jason Wang <jasowang@redhat.com>,
	qemu-level <qemu-devel@nongnu.org>,
	Daniel Daly <dandaly0@gmail.com>,
	virtualization@lists.linux-foundation.org,
	Liran Alon <liralon@gmail.com>, Eli Cohen <eli@mellanox.com>,
	Nitin Shrivastav <nitin.shrivastav@broadcom.com>,
	Alex Barba <alex.barba@broadcom.com>,
	Christophe Fontaine <cfontain@redhat.com>,
	Juan Quintela <quintela@redhat.com>,
	Lee Ballard <ballle98@gmail.com>,
	Lars Ganrot <lars.ganrot@gmail.com>,
	Rob Miller <rob.miller@broadcom.com>,
	Stefano Garzarella <sgarzare@redhat.com>,
	Howard Cai <howard.cai@gmail.com>,
	Parav Pandit <parav@mellanox.com>, vm <vmireyno@marvell.com>,
	Salil Mehta <mehta.salil.lnk@gmail.com>,
	Stephen Finucane <stephenfin@redhat.com>,
	Xiao W Wang <xiao.w.wang@intel.com>,
	Sean Mooney <smooney@redhat.com>,
	Jim Harford <jim.harford@broadcom.com>,
	Dmytro Kazantsev <dmytro.kazantsev@gmail.com>,
	Siwei Liu <loseweigh@gmail.com>,
	Harpreet Singh Anand <hanand@xilinx.com>,
	Michael Lilja <ml@napatech.com>, Max Gurtovoy <maxgu14@gmail.com>
Subject: Re: [RFC PATCH 13/27] vhost: Send buffers to device
Date: Thu, 10 Dec 2020 11:55:47 +0000	[thread overview]
Message-ID: <20201210115547.GH416119@stefanha-x1.localdomain> (raw)
In-Reply-To: <CAJaqyWf13ta5MtzmTUz2N5XnQ+ebqFPYAivdggL64LEQAf=y+A@mail.gmail.com>

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On Wed, Dec 09, 2020 at 07:41:23PM +0100, Eugenio Perez Martin wrote:
> On Tue, Dec 8, 2020 at 9:16 AM Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > On Fri, Nov 20, 2020 at 07:50:51PM +0100, Eugenio Pérez wrote:
> > > +        while (true) {
> > > +            int r;
> > > +            if (virtio_queue_full(vq)) {
> > > +                break;
> > > +            }
> >
> > Why is this check necessary? The guest cannot provide more descriptors
> > than there is ring space. If that happens somehow then it's a driver
> > error that is already reported in virtqueue_pop() below.
> >
> 
> It's just checked because virtqueue_pop prints an error on that case,
> and there is no way to tell the difference between a regular error and
> another caused by other causes. Maybe the right thing to do is just to
> not to print that error? Caller should do the error printing in that
> case. Should we return an error code?

The reason an error is printed today is because it's a guest error that
never happens with correct guest drivers. Something is broken and the
user should know about it.

Why is "virtio_queue_full" (I already forgot what that actually means,
it's not clear whether this is referring to avail elements or used
elements) a condition that should be silently ignored in shadow vqs?

Stefan

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