Re: [virtio-dev] queue_enable vs QueueReady

[Stefan Hajnoczi <stefanha@redhat.com>]

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?

I think MMIO is the outlier here. The device emulation code in QEMU
doesn't deal with queue shutdown. That only happens during device reset,
so it's like that a if the guest disables a virtio-mmio queue then
something undefined will happen on the QEMU side. For example, writing
used elements back to the virtqueue after it has been disabled.

If the VIRTIO spec really intends to support virtqueue shutdown then the
semantics need to be spelled out clearly: what happens to in-flight
requests? How do devices behave that rely on multiple virtqueues during
normal operation (e.g. virtio-vsock where you can't really have rx-only
or tx-only)?

Stefan

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