On Mon, Jan 27, 2025 at 08:44:50AM +0800, Jason Wang wrote: > On Sun, Jan 26, 2025 at 3:56 PM Jason Wang <jasow...@redhat.com> wrote: > > > > 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.
The major challenge when I was working on that (as far as I can still remember..): some devices are created at early stage of QEMU startup, which can happen before the vIOMMU is created and realized. Then it can be challenging to re-parent those devices to be childrens of the vIOMMU, or we may need a way to create vIOMMU always earlier than those.. > > Btw, I actually meant to break the assumption that vhost needs to be > enabled/disabled after/before vIOMMU. This only works for virtio-net / > vhost. From the view of vhost, it would work similar to _F_LOG_ALL > (where there's no assumption on the order of enabling/disabling dirty > page tracking and device start/stop). Yes, we can go for a lightweight solution. Thanks, -- Peter Xu
