Re: [PATCH] hw/virtio/vhost: Disable IOTLB callbacks when IOMMU gets disabled

[Jason Wang]

On Sat, Jan 25, 2025 at 12:42 AM Peter Xu <peterx@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-peterx@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
>