linux-kernel - Re: [virtio-dev] Re: [PATCH v11 6/6] virtio-balloon: VIRTIO_BALLOON_F_CMD

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Date:   Wed, 21 Jun 2017 15:28:56 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, david@...hat.com,
        dave.hansen@...el.com, cornelia.huck@...ibm.com,
        akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        aarcange@...hat.com, amit.shah@...hat.com, pbonzini@...hat.com,
        liliang.opensource@...il.com, riel@...hat.com, nilal@...hat.com
Subject: Re: [virtio-dev] Re: [PATCH v11 6/6] virtio-balloon:
 VIRTIO_BALLOON_F_CMD_VQ

On Wed, Jun 21, 2017 at 11:28:00AM +0800, Wei Wang wrote:
> On 06/21/2017 12:18 AM, Michael S. Tsirkin wrote:
> > On Fri, Jun 09, 2017 at 06:41:41PM +0800, Wei Wang wrote:
> > > -	if (!virtqueue_indirect_desc_table_add(vq, desc, num)) {
> > > +	if (!virtqueue_indirect_desc_table_add(vq, desc, *num)) {
> > >   		virtqueue_kick(vq);
> > > -		wait_event(vb->acked, virtqueue_get_buf(vq, &len));
> > > -		vb->balloon_page_chunk.chunk_num = 0;
> > > +		if (busy_wait)
> > > +			while (!virtqueue_get_buf(vq, &len) &&
> > > +			       !virtqueue_is_broken(vq))
> > > +				cpu_relax();
> > > +		else
> > > +			wait_event(vb->acked, virtqueue_get_buf(vq, &len));
> > 
> > This is something I didn't previously notice.
> > As you always keep a single buffer in flight, you do not
> > really need indirect at all. Just add all descriptors
> > in the ring directly, then kick.
> > 
> > E.g.
> > 	virtqueue_add_first
> > 	virtqueue_add_next
> > 	virtqueue_add_last
> > 
> > ?
> > 
> > You also want a flag to avoid allocations but there's no need to do it
> > per descriptor, set it on vq.
> > 
> 
> Without using the indirect table, I'm thinking about changing to use
> the standard sg (i.e. struct scatterlist), instead of vring_desc, so that
> we don't need to modify or add any new functions of virtqueue_add().
> 
> In this case, we will kmalloc an array of sgs in probe(), and we can add
> the sgs one by one to the vq, which won't trigger the allocation of an
> indirect table inside virtqueue_add(), and then kick when all are added.
> 
> Best,
> Wei

And allocate headers too? This can work. API extensions aren't
necessarily a bad idea though. The API I suggest above is preferable
for the simple reason that it can work without INDIRECT flag
support in hypervisor.

I wonder which APIs would Nitesh find useful.

-- 
MST