Linux Virtualization - Apr 2019 - [PATCH RFC 3/4] vsock/virtio: change the maximum packet size allowed

On Mon, Apr 08, 2019 at 10:37:23AM +0100, Stefan Hajnoczi
wrote:
> On Fri, Apr 05, 2019 at 12:07:47PM +0200, Stefano Garzarella wrote:
> > On Fri, Apr 05, 2019 at 09:24:47AM +0100, Stefan Hajnoczi wrote:
> > > On Thu, Apr 04, 2019 at 12:58:37PM +0200, Stefano Garzarella
wrote:
> > > > Since now we are able to split packets, we can avoid
limiting
> > > > their sizes to VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE.
> > > > Instead, we can use VIRTIO_VSOCK_MAX_PKT_BUF_SIZE as the max
> > > > packet size.
> > > > 
> > > > Signed-off-by: Stefano Garzarella <sgarzare at
redhat.com>
> > > > ---
> > > >  net/vmw_vsock/virtio_transport_common.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/net/vmw_vsock/virtio_transport_common.c
b/net/vmw_vsock/virtio_transport_common.c
> > > > index f32301d823f5..822e5d07a4ec 100644
> > > > --- a/net/vmw_vsock/virtio_transport_common.c
> > > > +++ b/net/vmw_vsock/virtio_transport_common.c
> > > > @@ -167,8 +167,8 @@ static int
virtio_transport_send_pkt_info(struct vsock_sock *vsk,
> > > >  	vvs = vsk->trans;
> > > >  
> > > >  	/* we can send less than pkt_len bytes */
> > > > -	if (pkt_len > VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE)
> > > > -		pkt_len = VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE;
> > > > +	if (pkt_len > VIRTIO_VSOCK_MAX_PKT_BUF_SIZE)
> > > > +		pkt_len = VIRTIO_VSOCK_MAX_PKT_BUF_SIZE;
> > > 
> > > The next line limits pkt_len based on available credits:
> > > 
> > >   /* virtio_transport_get_credit might return less than pkt_len
credit */
> > >   pkt_len = virtio_transport_get_credit(vvs, pkt_len);
> > > 
> > > I think drivers/vhost/vsock.c:vhost_transport_do_send_pkt() now
works
> > > correctly even with pkt_len > VIRTIO_VSOCK_MAX_PKT_BUF_SIZE.
> > 
> > Correct.
> > 
> > > 
> > > The other ->send_pkt() callback is
> > > net/vmw_vsock/virtio_transport.c:virtio_transport_send_pkt_work()
and it
> > > can already send any size packet.
> > > 
> > > Do you remember why VIRTIO_VSOCK_MAX_PKT_BUF_SIZE still needs to
be the
> > > limit?  I'm wondering if we can get rid of it now and just
limit packets
> > > to the available credits.
> > 
> > There are 2 reasons why I left this limit:
> > 1. When the host receives a packets, it must be <> >   
VIRTIO_VSOCK_MAX_PKT_BUF_SIZE [drivers/vhost/vsock.c:vhost_vsock_alloc_pkt()]
> >    So in this way we can limit the packets sent from the guest.
> 
> The general intent is to prevent the guest from sending huge buffers.
> This is good.
> 
> However, the guest must already obey the credit limit advertized by the
> host.  Therefore I think we should be checking against that instead of
> an arbitrary constant limit.
> 
> So I think the limit should be the receive buffer size, not
> VIRTIO_VSOCK_MAX_PKT_BUF_SIZE.  But at this point the code doesn't know
> which connection the packet is associated with and cannot check the
> receive buffer size. :(
> 
> Anyway, any change to this behavior requires compatibility so new guest
> drivers work with old vhost_vsock.ko.  Therefore we should probably just
> leave the limit for now.
I understood your point of view and I completely agree with you.
But, until we don't have a way to expose features/versions between guest
and host, maybe is better to leave the limit in order to be compatible
with old vhost_vsock.
> 
> > 2. When the host send packets, it help us to increase the parallelism
> >    (especially if the guest has 64 KB RX buffers) because the user
thread
> >    will split packets, calling multiple times
transport->stream_enqueue()
> >    in net/vmw_vsock/af_vsock.c:vsock_stream_sendmsg() while the
> >    vhost_transport_send_pkt_work() send them to the guest.
> 
> Sorry, I don't understand the reasoning.  Overall this creates more
> work.  Are you saying the benefit is that
> vhost_transport_send_pkt_work() can run "early" and notify the
guest of
> partial rx data before all of it has been enqueued?
Something like that. Your reasoning is more accurate.
Anyway, I'll do some tests in order to understand better the behaviour!

Thanks,
Stefano

On Mon, Apr 08, 2019 at 04:55:31PM +0200, Stefano Garzarella
wrote:
> > Anyway, any change to this behavior requires compatibility so new
guest
> > drivers work with old vhost_vsock.ko.  Therefore we should probably
just
> > leave the limit for now.
> 
> I understood your point of view and I completely agree with you.
> But, until we don't have a way to expose features/versions between
guest
> and host,
Why not use the standard virtio feature negotiation mechanism for this?
> maybe is better to leave the limit in order to be compatible
> with old vhost_vsock.
-- 
MST

On Mon, Apr 08, 2019 at 10:57:44AM -0400, Michael S. Tsirkin
wrote:
> On Mon, Apr 08, 2019 at 04:55:31PM +0200, Stefano Garzarella wrote:
> > > Anyway, any change to this behavior requires compatibility so new
guest
> > > drivers work with old vhost_vsock.ko.  Therefore we should
probably just
> > > leave the limit for now.
> > 
> > I understood your point of view and I completely agree with you.
> > But, until we don't have a way to expose features/versions between
guest
> > and host,
> 
> Why not use the standard virtio feature negotiation mechanism for this?
> 
Yes, I have this in my mind :), but I want to understand better if we can
use virtio-net also for this mechanism.
For now, I don't think limiting the packets to 64 KiB is a big issue.

What do you think if I postpone this when I have more clear if we can
use virtio-net or not? (in order to avoid duplicated work)

Thanks,
Stefano