[LSF/MM/BPF BOF] Userspace command aborts

[Hannes Reinecke]

On 2/17/23 19:53, Chaitanya Kulkarni wrote:
> On 2/16/23 08:40, Keith Busch wrote:
>> 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.
>>    
> 
> I'd avoid implementing userspace aborts and fix things in spec first.
> 
What's there to fix in the spec for aborts? You can't avoid the fact 
that aborts might be sent just at the time when the completion arrives ...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
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