Re: [LSF/MM/BPF BOF] Userspace command abouts

[Chaitanya Kulkarni <chaitanyak@nvidia.com>]

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.

-ck