Hello, Jason, Eric, On Fri, Jan 24, 2025 at 11:30:56AM +0800, Jason Wang wrote: > It might be because neither virtio bus nor virtio-net provides a > shutdown method. > > There used to be requests to provide those to unbreak the kexec. > > A quick try might be to provide a .driver.shutdown to > virtio_net_driver structure and reset the device there as a start.
I didn't check virtio driver path, but if that's missing it's reasonable to support it indeed. OTOH, even with that, vhost can still hit such DMA issue if it's a hard-reset, am I right? IOW, when using QMP command "system-reset". If my memory is correct, that's the problem I was working on the VFIO series, rather than a clean reboot. And that won't give guest driver chance to run anything, IIUC. I am wildly suspecting a VT-d write to GCMD to disable it can also appear if there's a hard reset, then when bootloading the VM the bios (or whatever firmware at early stage) may want to make sure the VT-d device is completely off by writting to GCMD. But that's a pure guess.. and that may or may not matter much on how we fix this problem. IOW, I suspect we need to fix both of them, (a) for soft-reset, by making sure drivers properly quiesce DMAs proactively when VM gracefully shuts down. (b) for hard-reset, by making sure QEMU reset in proper order. One thing to mention is for problem (b) VFIO used to have an extra challenge on !FLR devices, I discussed it in patch 4's comment there. Quotting from patch 4 of series: https://lore.kernel.org/all/20240117091559.144730-1-pet...@redhat.com * (1) Device depth-first reset hierachy doesn't yet work for vIOMMUs * (reference: resettable_cold_reset_fn()) * * Currently, vIOMMU devices are created as normal '-device' * cmdlines. It means in many ways it has the same attributes with * most of the rest devices, even if the rest devices should * logically be under control of the vIOMMU unit. * * One side effect of it is vIOMMU devices will be currently put * randomly under qdev tree hierarchy, which is the source of * device reset ordering in current QEMU (depth-first traversal). * It means vIOMMU now can be reset before some devices. For fully * emulated devices that's not a problem, because the traversal * holds BQL for the whole process. However it is a problem if DMA * can happen without BQL, like VFIO, vDPA or remote device process. * * TODO: one ideal solution can be that we make vIOMMU the parent * of the whole pci host bridge. Hence vIOMMU can be reset after * all the devices are reset and quiesced. * * (2) Some devices register its own reset functions * * Even if above issue solved, if devices register its own reset * functions for some reason via QEMU reset hooks, vIOMMU can still * be reset before the device. One example is vfio_reset_handler() * where FLR is not supported on the device. * * TODO: merge relevant reset functions into the device tree reset * framework. So maybe vhost doesn't have problem (2) listed above, and maybe it means it's still worthwhile thinking more about problem (1), which is to change the QOM tree to provide a correct topology representation when vIOMMU is present: so far it should be still a pretty much orphaned object there.. if QEMU relies on QOM tree topology for reset order, we may need to move it to the right place sooner or later. Thanks, -- Peter Xu
