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

[Thomas Gleixner]

On Fri, May 21 2021 at 09:31, Andy Lutomirski wrote:
> arch_prctl(SET_XSTATE_INIT_ON_FIRST_USE, TILE_STUFF);?
>
> As long as this is allowed to fail, I don’t have a huge problem with
> it.

I'm fine with that. It's still controlled by the OS and can return
-EPERM.

If allowed then the application would also accept to be insta killed if
that #NM allocation fails. Any bug report vs. that will be ignored.

> I think several things here are regrettable:
>
> 1. Legacy XSTATE code might assume that XCR0 is a constant.
>
> 2. Intel virt really doesn’t like us context switching XCR0, although
> we might say that this is Intel’s fault and therefore Intel’s
> problem. AMD hardware doesn’t appear to have this issue.
>
> 3. AMX bring tangled up in XSTATE is unfortunate.  The whole XSTATE
> mechanism is less than amazing.
>
> IMO the best we can make of this whole situation is to make XCR0
> dynamic, but the legacy compatibility issues are potentially
> problematic.

Why? The bit can be enabled and #NM catches the violation of the ABI
contract if the application did not request usage. No XCR0 fiddling on
context switch required.

Thanks,

        tglx