Re: [PATCH v5 28/28] x86/fpu/amx: Clear the AMX state when appropriate

[Len Brown]

On Mon, May 24, 2021 at 10:07 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:

> Could we maybe say:
>
>                 /*
>                  * Leaving state in the TILE registers may prevent the
>                  * processor from entering low-power idle states.  Use
>                  * TILERELEASE to initialize the state.  Destroying
>                  * fpregs state is safe after the fpstate update.
>                  */

Ack

> Also, referencing fpregs/fpstate is really nice because the codes
> doesn't actually say "XSAVE" anywhere.
>
> > +             if (fpu->state_mask & XFEATURE_MASK_XTILE_DATA)
> > +                     tile_release();
>
> Doesn't this tile_release() need a fpregs_deactivate()?  Otherwise, the
> next XRSTOR might get optimized away because it thinks there's still
> good data in the fpregs.
>
> Will this unnecessarily thwart the modified optimization in cases where
> we go and run this task again without ever going out to userspace?  Will
> this impact context-switch latency for *EVERY* context switch in order
> to go to a lower idle state in a few minutes, hours, or never?

yeah, seems we missed that.


thanks!
Len Brown, Intel Open Source Technology Center