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From: Thomas Gleixner <tglx@linutronix.de>
To: Dave Hansen <dave.hansen@intel.com>, Len Brown <lenb@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>, Willy Tarreau <w@1wt.eu>,
	Andy Lutomirski <luto@kernel.org>,
	Florian Weimer <fweimer@redhat.com>, "Bae\,
	Chang Seok" <chang.seok.bae@intel.com>, X86 ML <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	"libc-alpha\@sourceware.org" <libc-alpha@sourceware.org>,
	Rich Felker <dalias@libc.org>, Kyle Huey <me@kylehuey.com>,
	Keno Fischer <keno@juliacomputing.com>,
	Arjan van de Ven <arjan@linux.intel.com>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related features
Date: Fri, 21 May 2021 11:41:24 +0200	[thread overview]
Message-ID: <87bl94bf0b.ffs@nanos.tec.linutronix.de> (raw)
In-Reply-To: <9c8138eb-3956-e897-ed4e-426bf6663c11@intel.com>

Dave, Len,

On Thu, May 20 2021 at 15:53, Dave Hansen wrote:
> On 5/20/21 2:41 PM, Len Brown wrote:
>> So the questions are:
>> 1. who calls it -- a call/thread or process?  the application?  a
>> library -- which library?
>> 2. is it optional, or mandatory?
>> 3. if it is mandatory, what is the best way to enforce it?
>> 4. should we have a "release" system call too?
>> 
>> 1. Every thread needs a context switch buffer.  Does every thread make
>> the system call?  It seems sort of awkward for a library to always
>> make a system call before doing a TMUL.  It would be functionally
>> harmless, but it would add latency to an otherwise low-latency
>> operation.  If some central library does it, and caches that it has
>> done it before, then it would be ugly, but at least it would remove an
>> unnecessary user/kernel transition.
>
> Our system calls are *REALLY* fast.  We can even do a vsyscall for this
> if we want to get the overhead down near zero.  Userspace can also cache
> the "I did the prctl()" state in thread-local storage if it wants to
> avoid the syscall.

Correct.

>> 2. If it is optional, then v5 is code complete -- because it allows
>> you to allocate either explicitly via prtcl, or transparently via #NM.
>
> It needs to be mandatory.  If it's not, then nobody will use it, and
> they'll suffer the dreaded SIGSEGV-on-vmalloc()-failure and start filing
> bug reports.

Yes. Plus mandatory allows to do access control. IOW the prctl() can
return EPERM.

>> 3. If it is mandatory, then we should re-purpose the XFD mechanism:
>> app starts with XFD armed, by default
>> if app touches AMX before prctl, it takes a signal (and dies).

Yes.

>> When app calls prctl, allocate buffer disarm XFD for that app (exactly
>> what #NM trap does today).
>
> Yes, that sounds like a good use of XFD.

Agreed.

>> 4. I don't see a justification for a release concept, but it is
>> possible -- though sort of sticky with possible nested calls from
>> combinations of apps and libraries.  If that were sorted out by a
>> central library, then the actual system call on the last release per
>> thread would re-arm XFD to prevent access until the next explicit
>> request.  Unclear if it is important that the kernel actually do the
>> free -- some things might run faster if we keep it around...
>
> I think would be more of a get/put model rather than an allocate/free model.
>
> The "put" could effectively be a noop for now.

Yes.

> But, if we don't put this in the ABI up front, we can't add it later.
> That means that we could never add a lazy-free, even if we wanted to.

As I said somewhere in that thread, something like:

       prctl(PR_QUERY_XSTATE_FEATURES,....
       prctl(PR_ENABLE_XSTATE_FEATURES,....
       prctl(PR_DISABLE_XSTATE_FEATURES,....

To make this work you need refcounting and the last put (DISABLE) drops
the buffer and re-arms XFD. But of course an application/library can do
the put late if it knows that it's going to use it over and over.

Thanks,

        tglx

  reply	other threads:[~2021-05-21  9:41 UTC|newest]

Thread overview: 130+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-26 23:12 Candidate Linux ABI for Intel AMX and hypothetical new related features Andy Lutomirski
2021-03-26 23:18 ` Andy Lutomirski
2021-03-27  3:39   ` Len Brown
2021-03-27  9:14     ` Borislav Petkov
2021-03-27  9:58     ` Greg KH
2021-03-29 15:47       ` Len Brown
2021-03-29 16:38         ` Len Brown
2021-03-29 16:48           ` Florian Weimer
2021-03-29 18:14           ` Andy Lutomirski
2021-03-29 18:16         ` Andy Lutomirski
2021-03-29 22:38           ` Len Brown
2021-03-30  5:08             ` Andy Lutomirski
2021-03-30  5:50               ` Noah Goldstein
2021-03-30 17:01               ` Len Brown
2021-03-30 17:05                 ` Andy Lutomirski
2021-03-30 17:56                   ` Len Brown
2021-03-30 19:12                     ` Dave Hansen
2021-03-30 20:20                       ` Andy Lutomirski
2021-03-30 20:42                         ` Len Brown
2021-03-30 22:01                           ` David Laight
2021-03-31 16:31                             ` Len Brown
2021-03-31 16:53                               ` Andy Lutomirski
2021-03-31 21:42                                 ` Robert O'Callahan
2021-03-31 22:11                                   ` Len Brown
2021-03-31 22:28                                 ` Len Brown
2021-03-31 22:45                                   ` Andy Lutomirski
2021-04-09 20:52                                     ` Len Brown
2021-04-09 21:44                                       ` Andy Lutomirski
2021-04-11 19:07                                         ` Len Brown
2021-04-12  7:59                                           ` David Laight
2021-04-12 12:19                                           ` Borislav Petkov
2021-04-12 17:14                                           ` Sean Christopherson
2021-03-31 22:52                                   ` Borislav Petkov
2021-04-09 20:55                                     ` Len Brown
2021-03-28  0:53   ` Thomas Gleixner
2021-03-29  7:27     ` Peter Zijlstra
2021-03-29 15:06     ` Dave Hansen
2021-03-31  8:24 ` Borislav Petkov
     [not found] ` <87lf9nk2ku.fsf@oldenburg.str.redhat.com>
2021-04-12 14:31   ` Borislav Petkov
2021-04-12 14:38     ` Florian Weimer
2021-04-12 15:08       ` Borislav Petkov
2021-04-12 15:10       ` Andy Lutomirski
2021-04-12 15:21   ` Andy Lutomirski
2021-04-12 23:46     ` Len Brown
2021-04-13  0:17       ` Thomas Gleixner
2021-04-13  1:25         ` Len Brown
2021-04-13  3:43       ` Willy Tarreau
2021-04-13 19:51         ` Len Brown
2021-04-14  9:58           ` Borislav Petkov
2021-04-14 10:06             ` Willy Tarreau
2021-04-14 10:08               ` Borislav Petkov
2021-04-14 21:57             ` Len Brown
2021-04-15  4:43               ` Borislav Petkov
2021-04-15  5:29                 ` Willy Tarreau
2021-04-15  5:47                   ` Borislav Petkov
2021-04-16 22:05                     ` Len Brown
2021-04-19 14:14                       ` Borislav Petkov
2021-04-19 18:18                         ` Len Brown
2021-04-19 19:15                           ` Borislav Petkov
2021-04-19 21:33                             ` Len Brown
2021-04-19 21:58                               ` Borislav Petkov
2021-04-23 19:35                                 ` Len Brown
2021-04-23 19:57                                   ` Borislav Petkov
2021-05-02 15:27                                     ` Len Brown
2021-05-03  5:18                                       ` Florian Weimer
2021-05-03 13:43                                         ` Dave Hansen
2021-05-03 13:47                                           ` Florian Weimer
2021-05-03 14:14                                             ` Dave Hansen
2021-05-07 18:44                                           ` Thomas Gleixner
2021-05-07 18:50                                             ` Andy Lutomirski
2021-05-07 19:22                                               ` Thomas Gleixner
2021-05-08  9:45                                       ` Thomas Gleixner
2021-05-18 20:39                                         ` Len Brown
2021-05-19 23:29                                           ` Andy Lutomirski
2021-05-20 19:16                                             ` Len Brown
2021-05-17  9:45                                       ` Thomas Gleixner
2021-05-17  9:56                                         ` Florian Weimer
2021-05-17 10:18                                           ` Thomas Gleixner
2021-05-21 16:29                                           ` Len Brown
2021-05-17 13:49                                         ` Arjan van de Ven
2021-05-20 15:35                                         ` Len Brown
2021-05-20 20:54                                           ` Thomas Gleixner
2021-05-20 21:13                                             ` Dave Hansen
2021-05-20 21:41                                               ` Len Brown
2021-05-20 22:53                                                 ` Dave Hansen
2021-05-21  9:41                                                   ` Thomas Gleixner [this message]
2021-05-21 14:44                                                   ` Florian Weimer
2021-05-21 14:49                                                     ` Peter Zijlstra
2021-06-23 15:06                                                       ` Florian Weimer
2021-06-23 23:11                                                         ` Len Brown
2021-06-28 10:14                                                           ` Enrico Weigelt, metux IT consult
2021-06-28 12:49                                                             ` Florian Weimer
2021-06-30 12:22                                                               ` Enrico Weigelt, metux IT consult
2021-06-30 12:41                                                                 ` Willy Tarreau
2021-06-30 13:55                                                                 ` Arjan van de Ven
2021-06-30 15:20                                                                   ` Len Brown
2021-06-30 15:25                                                                   ` Enrico Weigelt, metux IT consult
2021-05-21 16:14                                                     ` Dave Hansen
2021-05-21 16:19                                                       ` Florian Weimer
2021-05-21 16:26                                                         ` Len Brown
2021-05-21 16:28                                                         ` Dave Hansen
2021-05-21 16:31                                                         ` Andy Lutomirski
2021-05-21 19:10                                                           ` Thomas Gleixner
2021-05-21 20:07                                                             ` Andy Lutomirski
2021-05-21 21:43                                                               ` Thomas Gleixner
2021-05-21 22:07                                                             ` Len Brown
2021-05-21 22:46                                                               ` Thomas Gleixner
2021-05-21 23:31                                                                 ` Len Brown
2021-05-22  7:16                                                                   ` Florian Weimer
2021-05-22 23:55                                                                     ` Andy Lutomirski
2021-05-21 23:06                                                               ` Dave Hansen
2021-05-21 23:08                                                                 ` Len Brown
2021-05-21 19:05                                                         ` Thomas Gleixner
2021-05-20 21:22                                             ` Len Brown
2021-05-20 21:41                                               ` Thomas Gleixner
2021-05-20 21:49                                                 ` Len Brown
2021-05-21  9:26                                                   ` Thomas Gleixner
2021-04-19 23:52                               ` Paul Eggert
2021-04-13 20:16       ` Andy Lutomirski
2021-04-13 22:47         ` Len Brown
2021-04-13 22:58           ` Andy Lutomirski
2021-04-14 21:48             ` Len Brown
2021-04-15 16:24               ` Andy Lutomirski
2021-04-15 17:00                 ` Dave Hansen
2021-04-15 17:38                   ` Andy Lutomirski
2021-04-16 21:54                 ` Len Brown
2021-04-16 22:03                   ` Andy Lutomirski
2021-04-16 22:10                     ` Len Brown
2021-04-16 22:14                       ` Andy Lutomirski
2021-04-17  1:57                         ` Len Brown

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