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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 28, 2021 at 08:08:41AM -0700, Thiago Macieira wrote:
> On Monday, 28 June 2021 05:40:32 PDT Enrico Weigelt, metux IT consult wrote:

> > What we SW engineers need is an easy and fast method to act depending on
> > whether some CPU supports some feature (eg. a new opcode). Things like
> > cpuinfo are only a tiny piece of that. What we could really use is a
> > conditional jump/call based on whether feature X is supported - without
> > any kernel intervention. Then the machine code could be easily layed out
> > to support both cases with our without some feature X. Alternatively we
> > could have a fast trapping in useland - hw generated call already would
> > be a big help.
> 
> 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.

> Yes, the checking became far more complex with the need to check XCR0 after 
> AVX came along, but since the instruction itself is a slow and serialising, 
> any library will just cache the results. And as a result, the level of CPU 
> features is not expected to change. It never has in the past, so this hasn't 
> been an issue.

Arguably you should be checking XCR0 for any feature there, including
SSE/AVX/AVX512 and now AMX.

Ideally we'd do a prctl() for AVX512 too, except it's too late :-(