Re: Locking between vfio hot-remove and pci sysfs sriov_numvfs

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On Thu, Dec 07, 2023 at 07:48:10PM -0400, Jason Gunthorpe wrote:
> 
> The mechanism of waiting in remove for userspace is inherently flawed,
> it can never work fully correctly. :( I've hit this many times.
> 
> Upon remove VFIO should immediately remove itself and leave behind a
> non-functional file descriptor. Userspace should catch up eventually
> and see it is toast.

One nice aspect of the current design is that vfio will leave the BARs
mapped until userspace releases the vfio handle. It avoids some rather
nasty hacks for handling SIGBUS errors in the fast path (i.e. writing
NVMe doorbells) where we cannot try to check for device removal on
every MMIO write. Would your proposal immediately yank the BARs, without
waiting for userspace to respond? This is mostly for my curiosity - SPDK
already has these hacks implemented, so I don't think it would be
affected by this kind of change in behavior.




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