From: Peter Zijlstra <peterz@infradead.org>
To: Thiago Macieira <thiago.macieira@intel.com>
Cc: fweimer@redhat.com, "Enrico Weigelt,
metux IT consult" <lkml@metux.net>,
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: Mon, 28 Jun 2021 19:11:16 +0200 [thread overview]
Message-ID: <20210628171115.GA13401@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <2094802.S4rhTtsRBG@tjmaciei-mobl1>
On Mon, Jun 28, 2021 at 09:13:29AM -0700, Thiago Macieira wrote:
> On Monday, 28 June 2021 08:27:24 PDT Peter Zijlstra wrote:
> > > That's what cpuid is for. With GCC function multi-versioning or equivalent
> > > manually-rolled solutions, you can get exactly what you're asking for.
> >
> > Right, lots of self-modifying code solutions there, some of which can be
> > linker driven, some not. In the kernel we use alternative() to replace
> > short code sequences depending on CPUID.
> >
> > Userspace *could* do the same, rewriting code before first execution is
> > fairly straight forward.
>
> Userspace shouldn't do SMC. It's bad enough that JITs without caching exist,
> but having pure paged code is better. Pure pages are shared as needed by the
> kernel.
I don't feel that strongly; if SMC gets you measurable performance
gains, go for it. If you're short on memory, buy more.
> All you need is a simple bit test. You can then either branch to different
> code paths or write to a function pointer so it'll go there directly the next
> time. You can also choose to load different plugins depending on what CPU
> features were found.
Both bit tests and indirect function calls suffer the extra memory load,
which is not free.
> Consequence: CPU feature checking is done *very* early, often before main().
For the linker based ones, yes. IIRC the ifunc() attribute is
particularly useful here.
next prev parent reply other threads:[~2021-06-28 17:11 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-23 15:04 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
[not found] ` <3c5c29e2-1b52-3576-eda2-018fb1e58ff9@metux.net>
2021-06-28 13:20 ` Peter Zijlstra
[not found] ` <534d0171-2cc5-cd0a-904f-cd3c499b55af@metux.net>
2021-06-30 15:36 ` Thiago Macieira
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 [this message]
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
[not found] ` <e07294c9-b02a-e1c5-3620-7fae7269fdf1@metux.net>
2021-06-30 14:34 ` Florian Weimer
[not found] ` <030f1462-2bf9-39bc-d620-6d9fbe454a27@metux.net>
2021-06-30 15:38 ` Florian Weimer
[not found] ` <4ba30cb7-6854-0691-fad6-4ca9ce674ac2@metux.net>
2021-07-01 8:21 ` Florian Weimer
[not found] ` <034dcf9b-1f8c-23ee-86a6-791122bc0f8c@metux.net>
2021-07-06 12:57 ` Florian Weimer
2021-06-30 15:29 ` Thiago Macieira
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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