Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

[Thomas Gleixner]

On Fri, May 07 2021 at 11:50, Andy Lutomirski wrote:

> On Fri, May 7, 2021 at 11:44 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>
>> On Mon, May 03 2021 at 06:43, Dave Hansen wrote:
>> > On 5/2/21 10:18 PM, Florian Weimer wrote:
>> >>> 5. If the feature is enabled in XCR0, the user happily uses it.
>> >>>
>> >>>     For AMX, Linux implements "transparent first use"
>> >>>     so that it doesn't have to allocate 8KB context switch
>> >>>     buffers for tasks that don't actually use AMX.
>> >>>     It does this by arming XFD for all tasks, and taking a #NM
>> >>>     to allocate a context switch buffer only for those tasks
>> >>>     that actually execute AMX instructions.
>> >> What happens if the kernel cannot allocate that additional context
>> >> switch buffer?
>> >
>> > Well, it's vmalloc()'d and currently smaller that the kernel stack,
>> > which is also vmalloc()'d.  While it can theoretically fail, if it
>> > happens you have bigger problems on your hands.
>>
>> Such a buffer allocation might also exceed a per process/cgroup
>> limitation. Anything else which is accounted happens in syscall context
>> which then returns an error on which the application can react.
>>
>> So what's the consequence when the allocation fails? Kill it right away
>> from #NM? Kill it on the first signal? Do nothing and see what happens?
>>
> It has to be an immediate signal or kill.

Killing it right there is the only sensible thing to do.

> A failure to load FPU state is somewhat tolerable (and has to be for
> CET), but a failure to *save* FPU state on a context switch would be a
> really nasty can of worms.

:)

> At the very least we will want arch_prctl(ARCH_ALLOCTE_XSTATE, mask)
> to allow HPC workloads to manually allocate the state and get an error
> code if it fails.

Yes.

Thanks,

        tglx