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 > _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
