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

[Jason Wang]

On 2019/3/21 下午4:44, Michael S. Tsirkin wrote:
> 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
> Any update here? Looks like a spec violation in qemu
> so I think we should fix this before we put out the
> release. Jason do you have the time to work on a fix?


Will post patches for Yuri to test soon.

Thanks


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