Hi Tamar, On Tue, Jul 15, 2025 at 07:00:02AM +0000, Tamar Christina wrote: > Hi Yury, > > Thanks for the clear comments!
Thanks goes to Richard! > > -----Original Message----- > > From: Yury Khrustalev <yury.khrusta...@arm.com> > > Sent: Thursday, June 19, 2025 2:40 PM > > To: gcc-patches@gcc.gnu.org > > Cc: Richard Sandiford <richard.sandif...@arm.com>; Mark Rutland > > <mark.rutl...@arm.com> > > Subject: [PATCH 1/1] aarch64: Adapt unwinder to linux's SME signal behaviour > > > > From: Richard Sandiford <richard.sandif...@arm.com> > > > > SME uses a lazy save system to manage ZA. The idea is that, > > if a function with ZA state wants to call a "normal" function, > > it can leave its state in ZA and instead set up a lazy save buffer. > > If, unexpectedly, that normal function contains a nested use of ZA, > > that nested use of ZA must commit the lazy save first. > > > > This lazy save system uses a special system register called TPIDR2_EL0. > > See: > > > > https://github.com/ARM-software/abi- > > aa/blob/main/aapcs64/aapcs64.rst#66the-za-lazy-saving-scheme > > > > for details. > > > > The ABI specifies that, on entry to an exception handler, the following > > things must be true: > > > > * PSTATE.SM must be 0 (the processor must be in non-streaming mode) > > > > * PSTATE.ZA must be 0 (ZA must be off) > > > > * TPIDR2_EL0 must be 0 (there must be no uncommitted lazy save) > > > > This is normally done by making _Unwind_RaiseException & friends > > commit any lazy save before they unwind. This also has the side > > effect of ensuring that TPIDR2_EL0 is never left pointing to a > > lazy save buffer that has been unwound. > > > > However, things get more complicated with signals. If: > > > > (a) a signal is raised while ZA is dormant (that is, while there is an > > uncommitted lazy save); > > > > (b) the signal handler throws an exception; and > > > > (c) that exception is caught outside the signal handler > > > > something must ensure that the lazy save from (a) is committed. > > > > This would be simple if the signal handler was entered with ZA and > > TPIDR2_EL0 intact. However, for various good reasons that are out > > of scope here, this is not done. Instead, Linux now clears both > > TPIDR2_EL0 and PSTATE.ZA before entering a signal handler, see: > > > > https://lore.kernel.org/all/20250417190113.3778111-1- > > mark.rutl...@arm.com/ > > > > for details. > > > > Therefore, it is the unwinder that must simulate a commit of the lazy > > save from (a). It can do this by reading the previous values of > > TPIDR2_EL0 and ZA from the sigcontext. > > > > The SME-related sigcontext structures were only added to linux's > > asm/sigcontext.h relatively recently and we can't rely on GCC being > > built against such recent kernel header files. The patch therefore uses > > defines relevant macros if they are not defined and provide types that > > comply with ABI layout of the corresponding linux types. > > > > The patch includes some ugly casting in an attempt to support big-endian > > ILP32, even though SME on big-endian ILP32 linux should never be a thing. > > We can remove it if we also remove ILP32 support from GCC. > > > > ILP32 is currently deprecated and we'll probably try to remove it this year. > Perhaps a middle ground is to wrap them in __ILP32__ so we know to find > them when we remove the support? I could certainly add #if defined (__ILP32__) for the type casting bits, if you think this is helpful for the future refactoring. > OK with that change, but give others a day or two to object. > > I assume this is something we'd want to backport? Yes, we plan to back-port it to GCC 15 and 14 (it's the first version where SME support was added to). Thanks, Yury
