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

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Date:   Tue, 30 Mar 2021 12:12:29 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Len Brown <lenb@...nel.org>, Andy Lutomirski <luto@...capital.net>
Cc:     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 3/30/21 10:56 AM, Len Brown wrote:
> On Tue, Mar 30, 2021 at 1:06 PM Andy Lutomirski <luto@...capital.net> wrote:
>>> On Mar 30, 2021, at 10:01 AM, Len Brown <lenb@...nel.org> wrote:
>>> Is it required (by the "ABI") that a user program has everything
>>> on the stack for user-space XSAVE/XRESTOR to get back
>>> to the state of the program just before receiving the signal?
>> The current Linux signal frame format has XSTATE in uncompacted format,
>> so everything has to be there.
>> Maybe we could have an opt in new signal frame format, but the details would need to be worked out.
>>
>> It is certainly the case that a signal should be able to be delivered, run “async-signal-safe” code,
>> and return, without corrupting register contents.
> And so an an acknowledgement:
> 
> We can't change the legacy signal stack format without breaking
> existing programs.  The legacy is uncompressed XSTATE.  It is a
> complete set of architectural state -- everything necessary to
> XRESTOR.  Further, the sigreturn flow allows the signal handler to
> *change* any of that state, so that it becomes active upon return from
> signal.

One nit with this: XRSTOR itself can work with the compacted format or
uncompacted format.  Unlike the XSAVE/XSAVEC side where compaction is
explicit from the instruction itself, XRSTOR changes its behavior by
reading XCOMP_BV.  There's no XRSTORC.

The issue with using the compacted format is when legacy software in the
signal handler needs to go access the state.  *That* is what can't
handle a change in the XSAVE buffer format (either optimized/XSAVEOPT,
or compacted/XSAVEC).