Re: [Qemu-devel] [PATCH] virtio-net: do not start queues that are not enabled by the guest

[Michael S. Tsirkin]

On Thu, Mar 14, 2019 at 10:45:25PM +0800, Jason Wang wrote:
> 
> On 2019/2/25 下午8:33, Michael S. Tsirkin wrote:
> > On Mon, Feb 25, 2019 at 03:47:48PM +0800, Jason Wang wrote:
> > > On 2019/2/22 下午12:22, Michael S. Tsirkin wrote:
> > > > On Thu, Feb 21, 2019 at 10:10:08PM -0500, Michael S. Tsirkin wrote:
> > > > > On Fri, Feb 22, 2019 at 11:04:05AM +0800, Jason Wang wrote:
> > > > > > On 2019/2/22 上午9:35, Michael S. Tsirkin wrote:
> > > > > > > On Thu, Feb 21, 2019 at 05:40:22PM +0800, Jason Wang wrote:
> > > > > > > > On 2019/2/21 下午4:18, Yuri Benditovich wrote:
> > > > > > > > 
> > > > > > > >            For 1.0 device, we can fix the queue_enable, but for 
> > > > > > > > 0.9x device how do
> > > > > > > >            you enable one specific queue in this case? (setting 
> > > > > > > > status?)
> > > > > > > > 
> > > > > > > > 
> > > > > > > >        Do I understand correctly that for 0.9 device in some 
> > > > > > > > cases the device will
> > > > > > > >        receive feature _MQ set, but will not receive 
> > > > > > > > VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET?
> > > > > > > >        Or the problem is different?
> > > > > > > > 
> > > > > > > > 
> > > > > > > > Let me clarify, VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET is used to 
> > > > > > > > control the the
> > > > > > > > number of queue pairs used by device for doing transmission and 
> > > > > > > > reception. It
> > > > > > > > was not used to enable or disable a virtqueue.
> > > > > > > > 
> > > > > > > > For 1.0 device, we should use queue_enable in pci cfg to enable 
> > > > > > > > and disable
> > > > > > > > queue:
> > > > > > > > 
> > > > > > > > 
> > > > > > > > We could do:
> > > > > > > > 
> > > > > > > > 1) allocate memory and set queue_enable for vq0
> > > > > > > > 
> > > > > > > > 2) allocate memory and set queue_enable for vq1
> > > > > > > > 
> > > > > > > > 3) Set vq paris to 1
> > > > > > > > 
> > > > > > > > 4) allocate memory and set queue_enable for vq2
> > > > > > > > 
> > > > > > > > 5) allocate memory and set queue_enable for vq3
> > > > > > > > 
> > > > > > > > 6) set vq pairs to 2
> > > > > > > I do not think spec allows this.
> > > > > > > 
> > > > > > > 
> > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > 1. Reset the device.
> > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the 
> > > > > > > device.
> > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the 
> > > > > > > device.
> > > > > > > 4. Read device feature bits, and write the subset of feature bits 
> > > > > > > understood by the OS and driver to the
> > > > > > > device. During this step the driver MAY read (but MUST NOT write) 
> > > > > > > the device-specific configuration
> > > > > > > fields to check that it can support the device before accepting 
> > > > > > > it.
> > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new 
> > > > > > > feature bits after this step.
> > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still 
> > > > > > > set: otherwise, the device does not
> > > > > > > support our subset of features and the device is unusable.
> > > > > > > 7. Perform device-specific setup, including discovery of 
> > > > > > > virtqueues for the device, optional per-bus setup,
> > > > > > > reading and possibly writing the device’s virtio configuration 
> > > > > > > space, and population of virtqueues.
> > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is 
> > > > > > > “live”.
> > > > > > > 
> > > > > > > 
> > > > > > > Thus vqs are setup at step 7.
> > > > > > > 
> > > > > > > # of vq pairs are set up through a command which is a special
> > > > > > > buffer, and spec says:
> > > > > > > 
> > > > > > > The driver MUST NOT send any buffer available notifications to 
> > > > > > > the device before setting DRIVER_OK.
> > > > > > So you meant write to queue_enable is forbidden after DRIVER_OK 
> > > > > > (though it's
> > > > > > not very clear to me from the  spec). And if a driver want to 
> > > > > > enable new
> > > > > > queues, it must reset the device?
> > > > > That's my reading.  What do you think?
> > > Looks like I can infer this from the spec, maybe it's better to clarify.
> > > 
> > > 
> > > > Btw some legacy drivers might violate this by addig buffers
> > > > before driver_ok.
> > > Yes, but it's probably too late to fix them.
> > > 
> > > Thanks.
> > Right. As a work around virtio net also checks rings
> > when it detects driver ok. We can disable that when VIRTIO_1
> > has been negotiated.
> > 
> 
> So, it looks to me the proper fix is probably start the vhost virtqueues
> selectively only when the vring address is not zero (for 0.9x) or
> queue_enable is set (for 1.0).
> 
> Thanks

Sounds good.

-- 
MST