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

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Date:   Wed, 31 Mar 2021 15:45:09 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Len Brown <lenb@...nel.org>
Cc:     David Laight <David.Laight@...lab.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        libc-alpha <libc-alpha@...rceware.org>,
        Florian Weimer <fweimer@...hat.com>,
        Rich Felker <dalias@...c.org>, Kyle Huey <me@...ehuey.com>,
        Keno Fischer <keno@...iacomputing.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

On Wed, Mar 31, 2021 at 3:28 PM Len Brown <lenb@...nel.org> wrote:
>
> On Wed, Mar 31, 2021 at 12:53 PM Andy Lutomirski <luto@...capital.net> 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.

I followed your link and found nothing. Can you elaborate?  In the
kernel, we have noinstr, and gcc gives approximately no help toward
catching problems.

>
> 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.

I don't think this is at all established.  If some code thinks it
knows the uncompacted XSTATE size and XCR0 changes, it crashes.  This
is not necessarily a showstopper.

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

This is true, it sucks, and Intel could fix it going forward.

>
> 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.

That is *a* goal, but not necessarily the only goal.

--Andy