RE: [PATCH vfio 11/11] vfio/virtio: Introduce a vfio driver over virtio devices

[Parav Pandit via Virtualization]

> From: Michael S. Tsirkin <m...@redhat.com>
> Sent: Tuesday, September 26, 2023 10:30 PM

> For example, a transitional device
> must not in theory be safely passed through to guest userspace, because guest
> then might try to use it through the legacy BAR without acknowledging
> ACCESS_PLATFORM.
> Do any guests check this and fail? Hard to say.
>
ACCESS_PLATFORM is not offered on the legacy interface because legacy interface 
spec 0.9.5 didn't have it.
Whether guest VM maps it to user space and using GIOVA is completely unknown to 
the device.
And all of this is just fine, because IOMMU through vfio takes care of 
necessary translation with/without mapping the transitional device to the guest 
user space.

Hence, it is not a compat problem.
Anyways, only those user will attach a virtio device to vfio-virtio device when 
user care to expose transitional device in guest.

I can see that in future, when user wants to do this optionally, a 
devlink/sysfs knob will be added, at that point, one needs to have a 
disable_transitional flag.
So it may be worth to optionally enable transitional support on user request as 
Michael suggested.
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