Re: x86 CPU features detection for applications (and AMX)

[Peter Zijlstra <peterz@infradead.org>]

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.