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On 6/2/24 08:38, Robert Clausecker wrote: > Greetings! > > I'm currently working on two projects that require the Zbb > instruction set extension on riscv64: > > - strajabot@ is a gsoc student who is currently porting the > SIMD-enhanced libc string functions from amd64 to riscv64. > For these, Zbb is immensely useful as it provides the > CTZ and ORC.B instructions needed for fast string comparison > > - I'm trying to write a fast sha1 implementation for libmd, > which (you might have guessed it) requires Zbb for rotate > instructions. Not having these is possible here, but there > is a significant performance penalty. > > Zbb is supported by the SiFive unmatched board for example. > > I originally wanted to just write an ifunc dispatcher that > checks for the presence off Zbb and then picks an appropriate > implementation, but then noticed that AT_HWCAP currently only > supports the detection of capital letter extensions. > Hi Robert, It should not be much work to start parsing for Zbb or similar extensions, so I can take this on soon. I mainly need to see what Linux is doing these days w.r.t. reporting multi-letter extensions to userspace. > Furthermore, we do not give user space any access to the isa > string (a sysctl like hw.isa would be nice), so it's not even > possible to determine the availability manually. > > Please check if you can add a sysctl to get the ISA string and > possibly add an elf_aux_info extension to allow the user to find > out which Z extensions are present. This is good feedback; possible a little more annoying to implement than it would seem, but I'll consider this too. Cheers, Mitchell > > We are working with 15-CURRENT, so if it's there it'll suffice > for our needs. > > Yours, > Robert Clausecker >
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