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 :-(
