[LSF/MM/BPF BOF] Userspace command abouts

[Keith Busch]

On Thu, Feb 16, 2023 at 12:50:03PM +0100, Hannes Reinecke wrote:
> Hi all,
> 
> it has come up in other threads, so it might be worthwhile to have its own
> topic:
> 
> Userspace command aborts
> 
> As it stands we cannot abort I/O commands from userspace.
> This is hitting us when running in a virtual machine:
> The VM sets a timeout when submitting a command, but that
> information can't be transmitted to the VM host. The VM host
> then issues a different command (with another timeout), and
> again that timeout can't be transmitted to the attached devices.
> So when the VM detects a timeout, it will try to issue an abort,
> but that goes nowhere as the VM host has no way to abort commands
> from userspace.
> So in the end the VM has to wait for the command to complete, causing
> stalls in the VM if the host had to undergo error recovery or something.

Aborts are racy. A lot of hardware implements these as a no-op, too.
 
> With io_uring or CDL we now have some mechanism which look as if they
> would allow us to implement command aborts.

CDL on the other hand sounds more promising.

> So this BoF will be around discussions on how aborts from userspace could be
> implemented, whether any of the above methods are suitable, or whether there
> are other ideas on how that could be done.