netdev - Re: [PATCH v10 25/41] virtio_pci: struct virtio_pci_common_cfg add queue_notify

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Message-ID: <20220627074723-mutt-send-email-mst@kernel.org>
Date:   Mon, 27 Jun 2022 07:53:22 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Mark Gross <markgross@...nel.org>,
        Vadim Pasternak <vadimp@...dia.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Cornelia Huck <cohuck@...hat.com>,
        Halil Pasic <pasic@...ux.ibm.com>,
        Eric Farman <farman@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        linux-um@...ts.infradead.org, netdev <netdev@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org,
        linux-remoteproc@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm <kvm@...r.kernel.org>,
        "open list:XDP (eXpress Data Path)" <bpf@...r.kernel.org>,
        kangjie.xu@...ux.alibaba.com
Subject: Re: [PATCH v10 25/41] virtio_pci: struct virtio_pci_common_cfg add
 queue_notify_data

On Mon, Jun 27, 2022 at 04:14:20PM +0800, Jason Wang wrote:
> On Mon, Jun 27, 2022 at 3:58 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > On Mon, Jun 27, 2022 at 03:45:30PM +0800, Jason Wang wrote:
> > > On Mon, Jun 27, 2022 at 2:39 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > >
> > > > On Mon, Jun 27, 2022 at 10:30:42AM +0800, Jason Wang wrote:
> > > > > On Fri, Jun 24, 2022 at 2:59 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > > > >
> > > > > > On Fri, Jun 24, 2022 at 10:56:05AM +0800, Xuan Zhuo wrote:
> > > > > > > Add queue_notify_data in struct virtio_pci_common_cfg, which comes from
> > > > > > > here https://github.com/oasis-tcs/virtio-spec/issues/89
> > > > > > >
> > > > > > > For not breaks uABI, add a new struct virtio_pci_common_cfg_notify.
> > > > > >
> > > > > > What exactly is meant by not breaking uABI?
> > > > > > Users are supposed to be prepared for struct size to change ... no?
> > > > >
> > > > > Not sure, any doc for this?
> > > > >
> > > > > Thanks
> > > >
> > > >
> > > > Well we have this:
> > > >
> > > >         The drivers SHOULD only map part of configuration structure
> > > >         large enough for device operation.  The drivers MUST handle
> > > >         an unexpectedly large \field{length}, but MAY check that \field{length}
> > > >         is large enough for device operation.
> > >
> > > Yes, but that's the device/driver interface. What's done here is the
> > > userspace/kernel.
> > >
> > > Userspace may break if it uses e.g sizeof(struct virtio_pci_common_cfg)?
> > >
> > > Thanks
> >
> > Hmm I guess there's risk... but then how are we going to maintain this
> > going forward?  Add a new struct on any change?
> 
> This is the way we have used it for the past 5 or more years. I don't
> see why this must be handled in the vq reset feature.
> 
> >Can we at least
> > prevent this going forward somehow?
> 
> Like have some padding?
> 
> Thanks

Maybe - this is what QEMU does ...

> >
> >
> > > >
> > > >
> > > >
> > > > >
> > > > > >
> > > > > >
> > > > > > > Since I want to add queue_reset after queue_notify_data, I submitted
> > > > > > > this patch first.
> > > > > > >
> > > > > > > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > > > > > > Acked-by: Jason Wang <jasowang@...hat.com>
> > > > > > > ---
> > > > > > >  include/uapi/linux/virtio_pci.h | 7 +++++++
> > > > > > >  1 file changed, 7 insertions(+)
> > > > > > >
> > > > > > > diff --git a/include/uapi/linux/virtio_pci.h b/include/uapi/linux/virtio_pci.h
> > > > > > > index 3a86f36d7e3d..22bec9bd0dfc 100644
> > > > > > > --- a/include/uapi/linux/virtio_pci.h
> > > > > > > +++ b/include/uapi/linux/virtio_pci.h
> > > > > > > @@ -166,6 +166,13 @@ struct virtio_pci_common_cfg {
> > > > > > >       __le32 queue_used_hi;           /* read-write */
> > > > > > >  };
> > > > > > >
> > > > > > > +struct virtio_pci_common_cfg_notify {
> > > > > > > +     struct virtio_pci_common_cfg cfg;
> > > > > > > +
> > > > > > > +     __le16 queue_notify_data;       /* read-write */
> > > > > > > +     __le16 padding;
> > > > > > > +};
> > > > > > > +
> > > > > > >  /* Fields in VIRTIO_PCI_CAP_PCI_CFG: */
> > > > > > >  struct virtio_pci_cfg_cap {
> > > > > > >       struct virtio_pci_cap cap;
> > > > > > > --
> > > > > > > 2.31.0
> > > > > >
> > > >
> >