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Tsirkin" To: Eugenio Perez Martin Cc: Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , qemu-devel@nongnu.org, Stefan Hajnoczi , Jason Wang Subject: Re: [PATCH 4/4] vhost: convert byte order on avail_event read Message-ID: <20221031083449-mutt-send-email-mst@kernel.org> References: <20221028160251.268607-1-eperezma@redhat.com> <20221028160251.268607-5-eperezma@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.048, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Qemu-devel" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org+qemu-devel=archiver.kernel.org@nongnu.org On Mon, Oct 31, 2022 at 09:29:53AM +0100, Eugenio Perez Martin wrote: > On Sat, Oct 29, 2022 at 12:53 AM Philippe Mathieu-Daudé > wrote: > > > > On 28/10/22 18:02, Eugenio Pérez wrote: > > > This causes errors on virtio modern devices on big endian hosts > > > > > > Signed-off-by: Eugenio Pérez > > > --- > > > hw/virtio/vhost-shadow-virtqueue.c | 3 ++- > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c > > > index 18a49e1ecb..3131903edd 100644 > > > --- a/hw/virtio/vhost-shadow-virtqueue.c > > > +++ b/hw/virtio/vhost-shadow-virtqueue.c > > > @@ -231,7 +231,8 @@ static void vhost_svq_kick(VhostShadowVirtqueue *svq) > > > size_t num = svq->vring.num; > > > uint16_t *avail_event = (uint16_t *)&svq->vring.used->ring[num]; > > > > > > > uint16_t avail_event = virtio_lduw_p(svq->vdev, > > &svq->vring.used->ring[num]); > > needs_kick = vring_need_event(avail_event, > > svq->shadow_avail_idx, > > svq->shadow_avail_idx - 1); > > > > It would work, but just because all vrings must be little endian for > the moment. If we support legacy drivers on a big endian host and > guest in the future, it would not work. > > virtio_ld and virtio_st handle the conversions between the guest and > the emulated device in qemu, but this conversion is between qemu > shadow vring and the vdpa device (assuming modern, little endian for > the moment). > > Right now the feature set must be the same, but it could not be that > way in the future. > > Thanks! I don't think this works legacy and virtio data path are similar but not similar enough to allow switches through svq alone. > > > - needs_kick = vring_need_event(*avail_event, svq->shadow_avail_idx, > > > + needs_kick = vring_need_event(le16_to_cpu(*avail_event), > > > + svq->shadow_avail_idx, > > > svq->shadow_avail_idx - 1); > > > } else { > > > needs_kick = !(svq->vring.used->flags & VRING_USED_F_NO_NOTIFY); > >