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Subject: device reset should entail QueuePFN reset (for all queues) on virtio-mmio
Cross-posting from the Linux Virtualization list [1] with minor edits: "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 is suboptimal per se, because a guest that crashes and reboots (while the emulator 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 Martin posted a series to edk2-devel [2] that adds virtio-mmio support to TianoCore, and Mark Salter tested it [3] on an AArch64 foundation model with a Linux guest, and found problems. Namely, the UEFI firmware can drive the virtio devices via virtio-mmio, but the Linux kernel booted from it can not. The reason is the missing zeroing of QueuePFN across ExitBootServices(). (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 the QueuePFNs, - 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 reset? Thanks! Laszlo [1] http://thread.gmane.org/gmane.linux.kernel.virtualization/21022 [2] http://thread.gmane.org/gmane.comp.bios.tianocore.devel/4557 [3] http://thread.gmane.org/gmane.comp.bios.tianocore.devel/4373/focus=4411
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