On 2023/4/28 02:32, Alex Williamson wrote:
On Thu, 27 Apr 2023 06:59:17 +0000
"Liu, Yi L" <yi.l.liu@xxxxxxxxx> wrote:
[...]
I'm not quite sure about it so far. For mdev devices, the device driver
may use vfio_pin_pages/vfio_dma_rw () to pin page. Hence such drivers
need to listen to dma_unmap() event. But for noiommu users, does the
device driver also participate in the page pin? At least for vfio-pci driver,
it does not, or maybe it will in the future when enabling noiommu
userspace to pin pages. It looks to me such userspace should order
the DMA before calling ioctl to unpin page instead of letting device
driver listen to unmap.
Whoa, noiommu is inherently unsafe an only meant to expose the vfio
device interface for userspace drivers that are going to do unsafe
things regardless. Enabling noiommu to work with mdev, pin pages, or
anything else should not be on our agenda. Userspaces relying on niommu
get the minimum viable interface and must impose a minuscule
incremental maintenance burden. The only reason we're spending so much
effort on it here is to make iommufd noiommu support equivalent to
group/container noiommu support. We should stop at that. Thanks,
btw. I asked a question in [1] to check if we should allow attach/detach
on noiommu devices. Jason has replied it. If in future noiommu userspace
can pin page, then such userspace will need to attach/detach ioas. So I
made cdev series[2] to allow attach ioas on noiommu devices. Supporting
it from cdev day-1 may avoid probing if attach/detach is supported or
not for specific devices when adding pin page for noiommu userspace.
But now, I think such a support will not in plan, is it? If so, will it
be better to disallow attach/detach on noiommu devices in patch [2]?
[1] https://lore.kernel.org/kvm/ZEa+khH0tUFStRMW@xxxxxxxxxx/
[2] https://lore.kernel.org/kvm/20230426150321.454465-21-yi.l.liu@xxxxxxxxx/
--
Regards,
Yi Liu
