Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 15 May 2023 00:45:24 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     davem@davemloft.net, edumazet@google.com, kuba@kernel.org,
        pabeni@redhat.com, virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, maxime.coquelin@redhat.com,
        alvaro.karsz@solid-run.com, eperezma@redhat.com,
        xuanzhuo@linux.alibaba.com, david.marchand@redhat.com,
        netdev <netdev@vger.kernel.org>
Subject: Re: [PATCH net-next V2 1/2] virtio-net: convert rx mode setting to
 use workqueue
Message-ID: <20230515004422-mutt-send-email-mst@kernel.org>
References: <20230413064027.13267-1-jasowang@redhat.com>
 <20230413064027.13267-2-jasowang@redhat.com>
 <20230413121525-mutt-send-email-mst@kernel.org>
 <CACGkMEunn1Z3n8yjVaWLqdV502yjaCBSAb_LO4KsB0nuxXmV8A@mail.gmail.com>
 <20230414031947-mutt-send-email-mst@kernel.org>
 <CACGkMEtutGn0CoJhoPHbzPuqoCLb4OCT6a_vB_WPV=MhwY0DXg@mail.gmail.com>
 <20230510012951-mutt-send-email-mst@kernel.org>
 <CACGkMEszPydzw_MOUOVJKBBW_8iYn66i_9OFvLDoZMH34hMx=w@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CACGkMEszPydzw_MOUOVJKBBW_8iYn66i_9OFvLDoZMH34hMx=w@mail.gmail.com>
Precedence: bulk

On Mon, May 15, 2023 at 09:05:54AM +0800, Jason Wang wrote:
> On Wed, May 10, 2023 at 1:33 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Mon, Apr 17, 2023 at 11:40:58AM +0800, Jason Wang wrote:
> > > On Fri, Apr 14, 2023 at 3:21 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > On Fri, Apr 14, 2023 at 01:04:15PM +0800, Jason Wang wrote:
> > > > > Forget to cc netdev, adding.
> > > > >
> > > > > On Fri, Apr 14, 2023 at 12:25 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > > >
> > > > > > On Thu, Apr 13, 2023 at 02:40:26PM +0800, Jason Wang wrote:
> > > > > > > This patch convert rx mode setting to be done in a workqueue, this is
> > > > > > > a must for allow to sleep when waiting for the cvq command to
> > > > > > > response since current code is executed under addr spin lock.
> > > > > > >
> > > > > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > > > >
> > > > > > I don't like this frankly. This means that setting RX mode which would
> > > > > > previously be reliable, now becomes unreliable.
> > > > >
> > > > > It is "unreliable" by design:
> > > > >
> > > > >       void                    (*ndo_set_rx_mode)(struct net_device *dev);
> > > > >
> > > > > > - first of all configuration is no longer immediate
> > > > >
> > > > > Is immediate a hard requirement? I can see a workqueue is used at least:
> > > > >
> > > > > mlx5e, ipoib, efx, ...
> > > > >
> > > > > >   and there is no way for driver to find out when
> > > > > >   it actually took effect
> > > > >
> > > > > But we know rx mode is best effort e.g it doesn't support vhost and we
> > > > > survive from this for years.
> > > > >
> > > > > > - second, if device fails command, this is also not
> > > > > >   propagated to driver, again no way for driver to find out
> > > > > >
> > > > > > VDUSE needs to be fixed to do tricks to fix this
> > > > > > without breaking normal drivers.
> > > > >
> > > > > It's not specific to VDUSE. For example, when using virtio-net in the
> > > > > UP environment with any software cvq (like mlx5 via vDPA or cma
> > > > > transport).
> > > > >
> > > > > Thanks
> > > >
> > > > Hmm. Can we differentiate between these use-cases?
> > >
> > > It doesn't look easy since we are drivers for virtio bus. Underlayer
> > > details were hidden from virtio-net.
> > >
> > > Or do you have any ideas on this?
> > >
> > > Thanks
> >
> > I don't know, pass some kind of flag in struct virtqueue?
> >         "bool slow; /* This vq can be very slow sometimes. Don't wait for it! */"
> >
> > ?
> >
> 
> So if it's slow, sleep, otherwise poll?
> 
> I feel setting this flag might be tricky, since the driver doesn't
> know whether or not it's really slow. E.g smartNIC vendor may allow
> virtio-net emulation over PCI.
> 
> Thanks

driver will have the choice, depending on whether
vq is deterministic or not.


> > --
> > MST
> >