On Mon, Feb 07, 2022 at 03:20:35PM +0000, Mark Brown wrote: > diff --git a/Documentation/arm64/sme.rst b/Documentation/arm64/sme.rst > new file mode 100644 > index 000000000000..15df3157c592 > --- /dev/null > +++ b/Documentation/arm64/sme.rst [...] > +3. Sharing of streaming and non-streaming mode SVE state > +--------------------------------------------------------- > + > +It is implementation defined which if any parts of the SVE state are shared > +between streaming and non-streaming modes. When switching between modes > +via software interfaces such as ptrace if no register content is provided as > +part of switching no state will be assumed to be shared and everything will > +be zeroed. Is there anything other than ptrace() here? I read the sigreturn() case below but did not say anything about changing PSTATE.SM via the sigcontext. I guess it's similar to ptrace(). > +4. System call behaviour > +------------------------- > + > +* On syscall PSTATE.ZA is preserved, if PSTATE.ZA==1 then the contents of the > + ZA matrix are preserved. Sorry if this was discussed. What is the rationale for preserving the ZA registers on syscall? We don't do this for the top part of the Z registers. > +* On syscall PSTATE.SM will be cleared and the SVE registers will be handled > + as normal. What does that mean? Is this as per the sve.rst doc (unspecified but zeroed in practice)? > +* Neither the SVE registers nor ZA are used to pass arguments to or receive > + results from any syscall. > + > +* On creation fork() or clone() the newly created process will have PSTATE.SM > + and PSTATE.ZA cleared. This looks slightly inconsistent with the first bullet point on ZA being preserved on syscalls. Why do these differ? [...] > +References > +========== > + > +[1] arch/arm64/include/uapi/asm/sigcontext.h > + AArch64 Linux signal ABI definitions > + > +[2] arch/arm64/include/uapi/asm/ptrace.h > + AArch64 Linux ptrace ABI definitions > + > +[3] Documentation/arm64/cpu-feature-registers.rst > + > +[4] ARM IHI0055C > + http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055c/IHI0055C_beta_aapcs64.pdf > + http://infocenter.arm.com/help/topic/com.arm.doc.subset.swdev.abi/index.html > + Procedure Call Standard for the ARM 64-bit Architecture (AArch64) The second link no longer works. I also couldn't find any reference to [4] but there's a lot of text to scan, so I may have missed it. -- Catalin
