Re: [PATCH 2/4] vhost-vdpa: reset vendor specific mapping to initial state in .release

[Si-Wei Liu]

On 10/17/2023 10:27 PM, Jason Wang wrote:
> > >    If we do
> > > this without a negotiation, IOTLB will not be clear but the Qemu will
> > > try to re-program the IOTLB after reset. Which will break?
> > >
> > > 1) stick the exact old behaviour with just one line of check
> > It's not just one line of check here, the old behavior emulation has to
> > be done as Eugenio illustrated in the other email.
> For vhost-vDPA it's just
>
> if (IOTLB_PERSIST is acked by userspace)
>      reset_map()
... and this reset_map in vhost_vdpa_cleanup can't be negotiable 
depending on IOTLB_PERSIST. Consider the case where user switches to 
virtio-vdpa after an older userspace using vhost-vdpa finished running. 
Even with buggy_virtio_reset_map in place it's unwarranted the vendor 
IOMMU can get back to the default state, e.g. ending with 1:1 
passthrough mapping. If not doing this unconditionally it will get a big 
chance to break userspace.

-Siwei

> For parent, it's somehow similar:
>
> during .reset()
>
> if (IOTLB_PERSIST is not acked by userspace)
>          reset_vendor_mappings()
>
> Anything I missed here?