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Subject: Re: [virtio-dev] Re: queue_enable vs QueueReady
On Tue, Jun 02, 2020 at 10:57:19AM +0800, Jason Wang wrote: > > On 2020/6/1 äå2:01, Michael S. Tsirkin wrote: > > On Thu, May 28, 2020 at 09:06:36PM +0800, Jason Wang wrote: > > > Hi: > > > > > > I found ambiguity in the virtio specification: > > > > > > In PCI part, it describes the queue_enable as: > > > > > > The driver uses this to selectively prevent the device from executing > > > requests from this virtqueue. 1 - enabled; 0 - disabled. > > > > > > In MMIO part, it describes the QueueReady as: > > > > > > Writing one (0x1) to this register notifies the device that it can execute > > > requests from this virtual queue. Reading from this register returns the > > > last value written to it. Both read and write accesses apply to the queue > > > selected by writing to QueueSel. > > > > > > If I understand this correctly, they have the same meaning, but the driver > > > requirements section looks conflict: > > > > > > PCI said: The driver MUST NOT write a 0 to queue_enable. > > > > > > MMIO said: > > > > > > To stop using the queue the driver MUST write zero (0x0) to this QueueReady > > > and MUST read the value back to ensure synchronization. > > > > > > So we can't disable a queue via queue_enable but QueueReady. Any reason for > > > such inconsistency? > > > > > > Thanks > > PCI assumed device reset is enough to stop all queues. We had tons of > > bugs around shutdown because of this, so in hindsight, MMIO had maybe a > > better idea. > > > > Ability to stop a queue and take back buffers would be nice, e.g. serial > > is kind of messed up around port disconnect without it. > > > Yes, but this inconsistency brings trouble for vDPA implementation which has > a set_queue_ready() and we do the following in virtio_vdpa as MMIO did: > > ÂÂÂ /* Select and deactivate the queue */ > ÂÂÂ ops->set_vq_ready(vdpa, index, 0); > ÂÂÂ WARN_ON(ops->get_vq_ready(vdpa, index)); > > The codes seems to work fine for IFC (a violation of the spec probably) but > not Qemu's virtio-net-pci. > > Thanks What do you mean "not Qemu's virtio-net-pci"? Which implementation of .set_vq_ready do you use? According to spec, I would expect this callback to do something other that writing into queue_enable. BTW I noticed this: if (cmd == VHOST_VDPA_SET_VRING_ENABLE) { if (copy_from_user(&s, argp, sizeof(s))) return -EFAULT; ops->set_vq_ready(vdpa, idx, s.num); return 0; } I'm guessing s.num should be 1, right? -- MST
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