From: Thiago Macieira <thiago.macieira@intel.com>
To: Peter Zijlstra <peterz@infradead.org>,
"Enrico Weigelt, metux IT consult" <lkml@metux.net>
Cc: <fweimer@redhat.com>, <hjl.tools@gmail.com>,
<libc-alpha@sourceware.org>, <linux-api@vger.kernel.org>,
<linux-arch@vger.kernel.org>, <x86@kernel.org>
Subject: Re: x86 CPU features detection for applications (and AMX)
Date: Wed, 30 Jun 2021 08:36:36 -0700 [thread overview]
Message-ID: <4315452.lxGN7TsiT6@tjmaciei-mobl1> (raw)
In-Reply-To: <534d0171-2cc5-cd0a-904f-cd3c499b55af@metux.net>
On Wednesday, 30 June 2021 05:50:30 PDT Enrico Weigelt, metux IT consult
wrote:
> > No, but because it's register state and part of XSAVE, it has immediate
> > impact in ABI. In particular, the signal stack layout includes XSAVE (as
> > does ptrace()).
>
> OMGs, I've already suspected such sickness. I don't even dare thinking
> about consequences for compilers and library ABIs.
>
> Does anyone here know why they designed this as inline operations ? This
> thing seems to be pretty much what typical TPUs are doing (or a subset
> of it). Why not just adding a TPU next to the CPU on the same chip ?
To be clear: this is a SW ABI. It has nothing to do the presence or absence of
other processing units in the system.
The moment you receive a Unix signal with SA_SIGINFO, the mcontext state needs
to be saved somewhere. Where would you save it? Please remember that:
- signal handlers can be called at any point in the execution, including
in the middle of malloc()
- signal handlers can longjmp out of the handler back into non-handler code
- in a multithreaded application, each thread can be handling a signal
simultaneously
We could have the kernel hold on to that and have a system call to extract
them, but that's an ABI change and I think won't work for the longjmp case.
> > Userspace will have to do something like:
> > - check CPUID, if !AMX -> fail
> > - issue prctl(), if error -> fail
> > - issue XGETBV and check the AMX bit it set, if not -> fail
>
> Can't we to this just by prctl() call ?
> IOW: ask the kernel, who gonna say yes or no.
That's possible. The kernel can't enable an AMX state on a system without AMX.
> Are there any situations where kernel says yes, but process still can't
> use it ? Why so ?
Today there is no such case that I can think of.
> > - request the signal stack size / spawn threads
>
> Signal stack is separate from the usual stack, right ?
> Why can't this all be done in one shot ?
Yes, we're talking about the sigaltstack() call.
What is "this all" in the sentence above?
--
Thiago Macieira - thiago.macieira (AT) intel.com
Software Architect - Intel DPG Cloud Engineering
next prev parent reply other threads:[~2021-06-30 15:36 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-23 15:04 x86 CPU features detection for applications (and AMX) Florian Weimer
2021-06-23 15:32 ` Dave Hansen
2021-07-08 6:05 ` Florian Weimer
2021-07-08 14:19 ` Dave Hansen
2021-07-08 14:31 ` Florian Weimer
2021-07-08 14:36 ` Dave Hansen
2021-07-08 14:41 ` Florian Weimer
2021-06-25 23:31 ` Thiago Macieira
2021-06-28 12:40 ` Enrico Weigelt, metux IT consult
2021-06-28 13:20 ` Peter Zijlstra
2021-06-30 12:50 ` Enrico Weigelt, metux IT consult
2021-06-30 15:36 ` Thiago Macieira [this message]
2021-07-01 7:35 ` Enrico Weigelt, metux IT consult
2021-06-28 15:08 ` Thiago Macieira
2021-06-28 15:27 ` Peter Zijlstra
2021-06-28 16:13 ` Thiago Macieira
2021-06-28 17:11 ` Peter Zijlstra
2021-06-28 17:23 ` Thiago Macieira
2021-06-28 19:08 ` Peter Zijlstra
2021-06-28 19:26 ` Thiago Macieira
2021-06-28 17:43 ` Peter Zijlstra
2021-06-28 19:05 ` Thiago Macieira
2021-06-30 14:32 ` Enrico Weigelt, metux IT consult
2021-06-30 14:34 ` Florian Weimer
2021-06-30 15:16 ` Enrico Weigelt, metux IT consult
2021-06-30 15:38 ` Florian Weimer
2021-07-01 8:08 ` Enrico Weigelt, metux IT consult
2021-07-01 8:21 ` Florian Weimer
2021-07-01 11:59 ` Enrico Weigelt, metux IT consult
2021-07-06 12:57 ` Florian Weimer
2021-06-30 15:29 ` Thiago Macieira
2021-07-01 11:57 ` Enrico Weigelt, metux IT consult
2021-07-08 7:08 ` Florian Weimer
2021-07-08 15:13 ` Thiago Macieira
2021-07-08 17:56 ` Mark Brown
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