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From: "Andy Lutomirski" <luto@kernel.org>
To: "Florian Weimer" <fweimer@redhat.com>,
	"Dave Hansen" <dave.hansen@intel.com>
Cc: "Dave Hansen via Libc-alpha" <libc-alpha@sourceware.org>,
	"Len Brown" <lenb@kernel.org>, "Rich Felker" <dalias@libc.org>,
	"Linux API" <linux-api@vger.kernel.org>,
	"Bae, Chang Seok" <chang.seok.bae@intel.com>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Kyle Huey" <me@kylehuey.com>, "Borislav Petkov" <bp@alien8.de>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Keno Fischer" <keno@juliacomputing.com>,
	"Arjan van de Ven" <arjan@linux.intel.com>,
	"Willy Tarreau" <w@1wt.eu>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related features
Date: Fri, 21 May 2021 09:31:26 -0700	[thread overview]
Message-ID: <263a58a9-26d5-4e55-b3e1-3718baf1b81d@www.fastmail.com> (raw)
In-Reply-To: <87r1i06ow2.fsf@oldenburg.str.redhat.com>



On Fri, May 21, 2021, at 9:19 AM, Florian Weimer wrote:
> * Dave Hansen:
> 
> > On 5/21/21 7:44 AM, Florian Weimer wrote:
> >> * Dave Hansen via Libc-alpha:
> >>> 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.
> >> Why can't userspace look at XCR0 to make the decision?
> >
> > The thing we're trying to avoid is a #NM exception from XFD (the new
> > first-use detection feature) that occurs on the first use of AMX.
> > XCR0 will have XCR0[AMX]=1, even if XFD is "armed" and ready to
> > generate the #NM.
> 
> I see.  So essentially the hardware wants to offer transparent
> initialize-on-use, but Linux does not seem to want to implement it this
> way.
> 
> Is there still a chance to bring the hardware and Linux into alignment?

arch_prctl(SET_XSTATE_INIT_ON_FIRST_USE, TILE_STUFF);?

As long as this is allowed to fail, I don’t have a huge problem with it.

I think several things here are regrettable:

1. Legacy XSTATE code might assume that XCR0 is a constant.

2. Intel virt really doesn’t like us context switching XCR0, although we might say that this is Intel’s fault and therefore Intel’s problem. AMD hardware doesn’t appear to have this issue.

3. AMX bring tangled up in XSTATE is unfortunate.  The whole XSTATE mechanism is less than amazing.

IMO the best we can make of this whole situation is to make XCR0 dynamic, but the legacy compatibility issues are potentially problematic.

> 
> Thanks,
> Florian
> 
> 

  parent reply	other threads:[~2021-05-21 16:31 UTC|newest]

Thread overview: 78+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20210415044258.GA6318@zn.tnic>
     [not found] ` <20210415052938.GA2325@1wt.eu>
     [not found]   ` <20210415054713.GB6318@zn.tnic>
     [not found]     ` <CAJvTdKnjzAMh3N_c7KP3kA=e0LgYHgCANg44oJp3LcSm7dtbSQ@mail.gmail.com>
     [not found]       ` <20210419141454.GE9093@zn.tnic>
     [not found]         ` <CAJvTdK=p8mgO3xw9sRxu0c7NTNTG109M442b3UZh8TqLLfkC1Q@mail.gmail.com>
     [not found]           ` <20210419191539.GH9093@zn.tnic>
     [not found]             ` <CAJvTdK=VnG94ECcRVoUi8HrCbVEKc8X4_JmRTkqe+vTttf0Wsg@mail.gmail.com>
     [not found]               ` <20210419215809.GJ9093@zn.tnic>
     [not found]                 ` <CAJvTdKn6JHo02karEs0e5g+6SimS5VUcXKjCkX35WY+xkgAgxw@mail.gmail.com>
     [not found]                   ` <YIMmwhEr46VPAZa4@zn.tnic>
     [not found]                     ` <CAJvTdKnhXnynybS4eNEF_EtF26auyb-mhKLNd1D9_zvCrchZsw@mail.gmail.com>
2021-05-17  9:45                       ` Candidate Linux ABI for Intel AMX and hypothetical new related features 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
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 [this message]
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
     [not found] <CALCETrW2QHa2TLvnUuVxAAheqcbSZ-5_WRXtDSAGcbG8N+gtdQ@mail.gmail.com>
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

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