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
next prev parent 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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