Sigh, cc linux-api, not linux-abi. On Fri, Mar 26, 2021 at 4:12 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote: > > Hi all- > > After some discussion on IRC, I have a proposal for a Linux ABI for > using Intel AMX and other similar features. It works like this: > > First, we make XCR0 dynamic. This looks a lot like Keno's patch but > with a different API, outlined below. Different tasks can have > different XCR0 values. The default XCR0 for new tasks does not > include big features like AMX. XMM and YMM are still there. The AVX2 > states are debatable -- see below. > > To detect features and control XCR0, we add some new arch_prctls: > > arch_prctl(ARCH_GET_XCR0_SUPPORT, 0, ...); > > returns the set of XCR0 bits supported on the current kernel. > > arch_prctl(ARCH_GET_XCR0_LAZY_SUPPORT, 0, ...); > > returns 0. See below. > > arch_prctl(ARCH_SET_XCR0, xcr0, lazy_states, sigsave_states, > sigclear_states, 0); > > Sets xcr0. All states are preallocated except that states in > lazy_states may be unallocated in the kernel until used. (Not > supported at all in v1. lazy_states & ~xcr0 != 0 is illegal.) States > in sigsave_states are saved in the signal frame. States in > sigclear_states are reset to the init state on signal delivery. > States in sigsave_states are restored by sigreturn, and states not in > sigsave_states are left alone by sigreturn. > > Optionally we do not support PKRU at all in XCR0 -- it doesn't make > that much sense as an XSAVE feature, and I'm not convinced that trying > to correctly context switch XINUSE[PKRU] is worthwhile. I doubt we > get it right today. > > Optionally we come up with a new format for new features in the signal > frame, since the current format is showing its age. Taking 8kB for a > signal with AMX is one thing. Taking another 8kB for a nested signal > if AMX is not in use is worse. > > Optionally we make AVX-512 also default off, which fixes what is > arguably a serious ABI break with AVX-512: lots of programs, following > POSIX (!), seem to think that they know much much space to allocate > for sigaltstack(). AVX-512 is too big. > > Thoughts? > > --Andy
