On Sat, Jan 25, 2025 at 12:42 AM Peter Xu <pet...@redhat.com> wrote: > > 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.
Yes. > > 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. Sounds like a non-trivial task, so for a hard reset, maybe we can proceed with Eric's proposal to deal with the reset before the device stops. Thanks > > Thanks, > > -- > Peter Xu >
