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Tue, 16 Feb 2021 05:39:12 -0500 X-MC-Unique: eCZlNYbpOfmgMFGcsIo2-A-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 3DB58184ECC4; Tue, 16 Feb 2021 10:39:11 +0000 (UTC) Received: from gondolin (ovpn-113-145.ams2.redhat.com [10.36.113.145]) by smtp.corp.redhat.com (Postfix) with ESMTP id 05DB660C15; Tue, 16 Feb 2021 10:39:09 +0000 (UTC) Date: Tue, 16 Feb 2021 11:39:07 +0100 From: Cornelia Huck To: Halil Pasic Subject: Re: [PATCH] virtio/s390: implement virtio-ccw revision 2 correctly Message-ID: <20210216113907.4e6943a9.cohuck@redhat.com> In-Reply-To: <20210215195144.7b96b41f.pasic@linux.ibm.com> References: <20210212170411.992217-1-cohuck@redhat.com> <20210215124702.23a093b8.cohuck@redhat.com> <20210215195144.7b96b41f.pasic@linux.ibm.com> Organization: Red Hat GmbH MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Cc: linux-s390@vger.kernel.org, kvm@vger.kernel.org, Pierre Morel , virtualization@lists.linux-foundation.org X-BeenThere: virtualization@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux virtualization List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: virtualization-bounces@lists.linux-foundation.org Sender: "Virtualization" On Mon, 15 Feb 2021 19:51:44 +0100 Halil Pasic wrote: > On Mon, 15 Feb 2021 12:47:02 +0100 > Cornelia Huck wrote: > > > On Fri, 12 Feb 2021 18:04:11 +0100 > > Cornelia Huck wrote: > > > > > CCW_CMD_READ_STATUS was introduced with revision 2 of virtio-ccw, > > > and drivers should only rely on it being implemented when they > > > negotiated at least that revision with the device. > > > > > > However, virtio_ccw_get_status() issued READ_STATUS for any > > > device operating at least at revision 1. If the device accepts > > > READ_STATUS regardless of the negotiated revision (which it is > > > free to do), > > > > So, looking at the standard again, the device is actually required to > > reject the READ_STATUS if only rev 1 had been negotiated... regardless > > of that, I don't think we should change QEMU's behaviour, as it would > > affect existing guests (they would lose access to the status bits as > > observed by the device, including DEVICE_NEEDS_RESET.) > > Not only that, without READ_STATUS, we can't do device reset which > is a prerequisite for a proper cleanup, as required by the spec. > > You certainly remember, the driver has may not assume the reset > was performed (and thus virtqueues are not live) until it reads > back status 0. But without READ_STATUS virtio_ccw_get_status() will > keep returning the status the driver last set via > virtio_ccw_set_status(). And CCW_CMD_VDEV_RESET is of course > revision 1 material. This looks ugly! Yes, that problem kind of cascades down. > > > > > > everything works as intended; a device rejecting the > > > command should also be handled gracefully. For correctness, we > > > should really limit the command to revision 2 or higher, though. > > > > > > We also negotiated the revision to at most 1, as we never bumped > > > the maximum revision; let's do that now. > > > > > > Fixes: 7d3ce5ab9430 ("virtio/s390: support READ_STATUS command for virtio-ccw") > > > Signed-off-by: Cornelia Huck > > > --- > > > > > > QEMU does not fence off READ_STATUS for revisions < 2, which is probably > > > why we never noticed this. I'm not aware of other hypervisors that do > > > fence it off, nor any that cannot deal properly with an unknown command. > > > > > > Not sure whether this is stable worthy? > > > > Maybe it is, given the MUST reject clause in the standard? > > > > Yes, IMHO this must be backported. A device that ain't violating the > spec would currently reject READ_STATUS. Which would break RESET_VDEV > like I described above. > > Can we change that MUST to should? There are now good reasons for not > doing like the spec says in case of READ_STATUS. Yes. I'm not so sure forcing the device to reject the command was such a good idea anyway, and relaxing the requirement keeps existing implementations in compliance. I've opened https://github.com/oasis-tcs/virtio-spec/issues/96 and will send a patch for the spec later. > > > > > > > --- > > > drivers/s390/virtio/virtio_ccw.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c > > > index 5730572b52cd..54e686dca6de 100644 > > > --- a/drivers/s390/virtio/virtio_ccw.c > > > +++ b/drivers/s390/virtio/virtio_ccw.c > > > @@ -117,7 +117,7 @@ struct virtio_rev_info { > > > }; > > > > > > /* the highest virtio-ccw revision we support */ > > > -#define VIRTIO_CCW_REV_MAX 1 > > > +#define VIRTIO_CCW_REV_MAX 2 > > > > > > struct virtio_ccw_vq_info { > > > struct virtqueue *vq; > > > @@ -952,7 +952,7 @@ static u8 virtio_ccw_get_status(struct virtio_device *vdev) > > > u8 old_status = vcdev->dma_area->status; > > > struct ccw1 *ccw; > > > > > > - if (vcdev->revision < 1) > > > + if (vcdev->revision < 2) > > > return vcdev->dma_area->status; > > I don't think our faking of the status read (i.e. returning the old one) > is contributing to spec compliance. Especially not if the inability to > READ is not transient. > > Also return old_status; would tell the story better, but on the > other hand, that would be an unrelated cosmetic change. Maybe > a separate patch? We would also need to actively check for success or failure of the channel program in that case. I'm currently looking at the virtio-ccw code anyway, so I can put that on my list as well. > > Reviewed-by: Halil Pasic Thanks! I'll do a v2 with a tweaked commit message and cc:stable. > > Regards, > Halil > > > > > > > ccw = ccw_device_dma_zalloc(vcdev->cdev, sizeof(*ccw)); > > > _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization