Re: [RFC PATCH 1/5] PCI: Refuse function reset of an SR-IOV PF with enabled VFs

On Fri, Aug 14, 2026 at 08:37:37AM -0600, Alex Williamson wrote:

> Devices are only ever opened into a user owned domain, the IOMMU
> context switch happens before this and regardless of the reset.  Close
> also disables bus-master regardless of reset, so there's no risk of
> ongoing DMA if the device is placed into an identity domain between
> close and re-open.

I've been told by HW people that bus-master isn't actually a strong
fence for stopping DMA on most real HW. More of a 'dma will eventually
stop' kind of thing.

It does reliably prevent DMA during the post-FLR phase only.

If the device hasn't been reset it should be kept on a blocking
domain.

> PF, I think there are arguments both that the user implicitly opted in
> to the best-effort reset, as well as a use case that allows the PF
> driver to fail and re-open the PF demands this behavior.

I really think we cannot alow a device to be returned back to the
kernel without a reset. VFIO should be keeping things attached to a
blocking domain as long as the driver is bound. It should force a
reset during unbinding as well if the device FD was ever closed
without a reset..

Jason