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

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Date:   Fri, 9 Apr 2021 16:52:50 -0400
From:   Len Brown <lenb@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     David Laight <David.Laight@...lab.com>,
        Dave Hansen <dave.hansen@...el.com>,
        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 6:45 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> On Wed, Mar 31, 2021 at 3:28 PM Len Brown <lenb@...nel.org> wrote:
>
> > 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.

A search for the word "interrupt" on this page
https://gcc.gnu.org/onlinedocs/gcc/x86-Function-Attributes.html#x86-Function-Attributes
comes to the description of this attribute:

__attribute__ ((interrupt))

> > 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.

My working assumption is that crashing applications actually *is* a showstopper.
Please clarify.

> > 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.

What hardware fix do you suggest?
If a guest is permitted to set XCR0 bits without notifying the VMM,
what happens when it sets bits that the VMM doesn't know about?

> > 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.

I fully support coming up with a scheme for fast future-proof signal handlers,
and I'm willing to back that up by putting work into it.

I don't see any other goals articulated in this thread.

thanks,
Len Brown, Intel Open Source Technology Center