netdev - Re: [RFC PATCH 07/13] vsock: handle buffer

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Date:   Fri, 11 Oct 2019 10:51:58 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     Stefan Hajnoczi <stefanha@...il.com>, netdev@...r.kernel.org,
        Sasha Levin <sashal@...nel.org>, linux-hyperv@...r.kernel.org,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        kvm@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
        Dexuan Cui <decui@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        "David S. Miller" <davem@...emloft.net>,
        Jorgen Hansen <jhansen@...are.com>
Subject: Re: [RFC PATCH 07/13] vsock: handle buffer_size sockopts in the core

On Fri, Oct 11, 2019 at 09:27:14AM +0100, Stefan Hajnoczi wrote:
> On Thu, Oct 10, 2019 at 11:32:54AM +0200, Stefano Garzarella wrote:
> > On Wed, Oct 09, 2019 at 01:30:26PM +0100, Stefan Hajnoczi wrote:
> > > On Fri, Sep 27, 2019 at 01:26:57PM +0200, Stefano Garzarella wrote:
> > > Another issue is that this patch drops the VIRTIO_VSOCK_MAX_BUF_SIZE
> > > limit that used to be enforced by virtio_transport_set_buffer_size().
> > > Now the limit is only applied at socket init time.  If the buffer size
> > > is changed later then VIRTIO_VSOCK_MAX_BUF_SIZE can be exceeded.  If
> > > that doesn't matter, why even bother with VIRTIO_VSOCK_MAX_BUF_SIZE
> > > here?
> > > 
> > 
> > The .notify_buffer_size() should avoid this issue, since it allows the
> > transport to limit the buffer size requested after the initialization.
> > 
> > But again the min set by the user can not be respected and in the
> > previous implementation we forced it to VIRTIO_VSOCK_MAX_BUF_SIZE.
> > 
> > Now we don't limit the min, but we guarantee only that vsk->buffer_size
> > is lower than VIRTIO_VSOCK_MAX_BUF_SIZE.
> > 
> > Can that be an acceptable compromise?
> 
> I think so.
> 
> Setting buffer sizes was never tested or used much by userspace
> applications that I'm aware of.  We should probably include tests for
> changing buffer sizes in the test suite.

Good idea! We should add a test to check if min/max are respected,
playing a bit with these sockopt.

I'll do it in the test series!

Thanks,
Stefano