linux-kernel - Re: [PATCH v4 4/5] vhost/vsock: split packets to send using multiple buffers

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Date:   Thu, 18 Jul 2019 04:13:26 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        virtualization@...ts.linux-foundation.org,
        Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v4 4/5] vhost/vsock: split packets to send using multiple
 buffers

On Thu, Jul 18, 2019 at 09:50:14AM +0200, Stefano Garzarella wrote:
> On Wed, Jul 17, 2019 at 4:55 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > On Wed, Jul 17, 2019 at 01:30:29PM +0200, Stefano Garzarella wrote:
> > > If the packets to sent to the guest are bigger than the buffer
> > > available, we can split them, using multiple buffers and fixing
> > > the length in the packet header.
> > > This is safe since virtio-vsock supports only stream sockets.
> > >
> > > Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
> >
> > So how does it work right now? If an app
> > does sendmsg with a 64K buffer and the other
> > side publishes 4K buffers - does it just stall?
> 
> Before this series, the 64K (or bigger) user messages was split in 4K packets
> (fixed in the code) and queued in an internal list for the TX worker.
> 
> After this series, we will queue up to 64K packets and then it will be split in
> the TX worker, depending on the size of the buffers available in the
> vring. (The idea was to allow EWMA or a configuration of the buffers size, but
> for now we postponed it)

Got it. Using workers for xmit is IMHO a bad idea btw.
Why is it done like this?

> Note: virtio-vsock only supports stream socket for now.
> 
> Thanks,
> Stefano