Re: [Qemu-devel] QueuePFN peculiarity in virtio-mmio

[Laszlo Ersek]

My apologies, I used Anthony's previous (now obsolete) email. Updated it
now & keeping full context below. Sorry.


On 10/22/13 19:49, Laszlo Ersek wrote:
> Hi,
> 
> "Appendix X: virtio-mmio" in the virtio spec says
> 
>     • 0x040 | RW | QueuePFN
>       [...] When the Guest stops using the queue it must write zero
>       (0x0) to this register.
>       [...]
> 
> and
> 
>     Virtqueue Configuration
> 
>     [...]
>     2. Check if the queue is not already in use: read QueuePFN
>     register, returned value should be zero (0x0).
>     [...]
> 
> I think this in itself is already suboptimal, because a guest that
> crashes and reboots (while the emulator itself survives) will not be
> able to use the device after said reboot (it has never re-set QueuePFN
> to zero).
> 
> But, more importantly: I think that resetting the device (by writing 0
> to its status register) should include (ie. *guarantee*) the effects of
> setting QueuePFN to zero for all imaginable queues of the device.
> 
> This way, a defensive guest that starts up by resetting the device (*)
> after identifying it via MagicValue / Version / DeviceID / VendorID
> would be able to use the device regardless of the device's prior
> QueuePFN setting(s).
> 
> (*) Resetting the device is the first step in "2.2.1 Device
> Initialization Sequence". It "is not required on initial start up", but
> as a guest driver can never be sure whether the startup in question is
> the initial one, a defensive driver will always start with device reet.
> 
> 
> The question arises because Olivier has posted a series to edk2-devel
> that adds virtio-mmio support to TianoCore, and Mark tested it (using
> OVMF) with a Linux guest and found problems. Namely, OVMF itself can
> drive the virtio devices via virtio-mmio, but the Linux kernel booted
> from OVMF can not. The reason is the missing zeroing of QueuePFN when
> OVMF is exiting. (I'm just paraphrasing the analysis.)
> 
> I think
> - that resetting the device (via its status register) should make the
> host forget *all* prior configuration, including QueuePFN,
> - and that the Linux driver should reset the device as first step.
> 
> So:
> - What's the motivation for the "acquire/release" semantics of QueuePFN?
> - Am I right that device reset should force a QueuePFN release too?
> 
> Thanks,
> Laszlo
>