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

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Dec 08, 2023 at 05:38:51PM +0000, Jim Harris wrote:
> 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.

What we did in RDMA was map a dummy page to the BARs so the sigbus was
avoided. But in that case RDMA knows the BAR memory is used only for
doorbell write so this is a reasonable thing to do.

Jason