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

[Andy Lutomirski]

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.  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.

--Andy