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

[Len Brown]

On Wed, Mar 31, 2021 at 12:53 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:

> But this whole annotation thing will require serious compiler support.
> We already have problems with compilers inlining functions and getting confused about attributes.

We added compiler annotation for user-level interrupt handlers.
I'm not aware of it failing, or otherwise being confused.

Why would compiler support for fast-signals be any more "serious"?

> An API like:
>
> if (get_amx()) {
>  use AMX;
> } else {
>  don’t;
> }
>
> Avoids this problem. And making XCR0 dynamic, for all its faults, at least helps force a degree of discipline on user code.

dynamic XCR0 breaks the installed base, I thought we had established that.

We've also established that when running in a VMM, every update to
XCR0 causes a VMEXIT.

I thought the goal was to allow new programs to have fast signal handlers.
By default, those fast signal handlers would have a stable state
image, and would
not inherit large architectural state on their stacks, and could thus
have minimal overhead on all hardware.