[PATCH RFC v2] RISCV: insert zimop instruction at the start

[enh]

On Fri, Jun 13, 2025 at 12:42 AM Deepak Gupta <debug@rivosinc.com> wrote:
>
> RVA23 mandates that CPU must implement zimop and zcmop extensions. These
> extensions are termed as "may be operations" to signify that some instructions
> might be leveraged by some future extensions to turn into different operations.
> Unless those extensions are used for codegen, there is no way to ensure that
> compiled software has zimop/zcmop instruction in them. Without any software
> using these instructions, there is no way to ensure that target hardware is
> RVA23 compatible.
>
> zicfiss extension converts some of the zimop instructions into shadow stack
> instructions. Support for shadow stack is still in the flux. This patch ensures
> that if compiler supports shadow stack codegen then sspush/sspopchk
> instructions are at the start when userspace starts life. Kernel doesn't enable
> shadow stack for userspace by default. Userspace will issue shadow stack enable
> prtctls to kernel. Thus always these (sspush/sspopchk) instructions will
> default to zimop behavior.
>
> Note that this is not a backward hardware compatible change.
>
> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
> ---
> RVA23 mandates that CPU must implement zimop and zcmop extensions. These
> extensions are termed as "may be operations" to signify that some instructions
> might be leveraged by some future extensions to turn into different operations.
> Unless those extensions are used for codegen, there is no way to ensure that
> compiled software has zimop/zcmop instruction in them. Without any software
> using these instructions, there is no way to ensure that target hardware is
> RVA23 compatible.
>
> zicfiss extension converts some of the zimop instructions into shadow stack
> instructions. Support for shadow stack is still in the flux. This patch ensures
> that if compiler supports shadow stack codegen then sspush/sspopchk
> instructions are at the start when userspace starts life. Kernel doesn't enable
> shadow stack for userspace by default. Userspace will issue shadow stack enable
> prctls to kernel. Thus always these (sspush/sspopchk) instructions will
> default to zimop behavior.
>
> Note that this is not a backward hardware compatible change.
> ---
> Changes in v2:
> - changing sspop to c.sspopchk. sspopchk is the instruction. making it
>   c.sspopchk to check for zcmop.
>
> - Link to v1: https://inbox.sourceware.org/20250612-glibc_zimop-v1-1-88f15a37d233@rivosinc.com
> ---
>  sysdeps/riscv/dl-machine.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/sysdeps/riscv/dl-machine.h b/sysdeps/riscv/dl-machine.h
> index dcc3e0883b..9870320cf0 100644
> --- a/sysdeps/riscv/dl-machine.h
> +++ b/sysdeps/riscv/dl-machine.h
> @@ -53,6 +53,15 @@
>       || (__WORDSIZE == 64 && (type) == R_RISCV_TLS_TPREL64)))  \
>     | (ELF_RTYPE_CLASS_COPY * ((type) == R_RISCV_COPY)))
>
> +
> +#ifdef __riscv_zicfiss

doesn't the #ifdef make this useless? you'll only emit these
instructions if you've enabled ss in your build (and thus presumably
emitting them for real), so you're not really testing the "maybe" side
of things at all?

don't you actually want to have a couple of unconditional `.inst`s for
any build claiming zimop/zicmop instead?

and even if you do that, is this a good place to do it? crashes here
tend to be hard to interpret/debug, especially without a SIGILL signal
handler to say "hey, you're running a build that requires zimop on a
non-zimop device".

does __riscv_hwprobe() for zimop make more sense?

(full disclosure: i have a todo to do something like this for bionic,
but haven't really worked out _what_ exactly. and i think the bionic
case is easier because we have no intention of supporting pre-zimop
devices, whereas i assume glibc wants to support everything? i guess
that's the real question before answering any of the rest: "what exact
set of combinations does glibc intend to support [and how is that
going to work in terms of being able to roll out ss if zimop can't be
assumed]?")

> +#define INSERT_ZIMOP \
> +     sspush x1       \
> +     c.sspopchk  x1
> +#else
> +#define INSERT_ZIMOP
> +#endif
> +
>  /* Return nonzero iff ELF header is compatible with the running host.  */
>  static inline int __attribute_used__
>  elf_machine_matches_host (const ElfW(Ehdr) *ehdr)
> @@ -102,6 +111,7 @@ elf_machine_dynamic (void)
>  #define RTLD_START asm (\
>         ".text\n\
>         " _RTLD_PROLOGUE (ENTRY_POINT) "\
> +        INSERT_ZIMOP \
>         mv a0, sp\n\
>         jal _dl_start\n\
>         " _RTLD_PROLOGUE (_dl_start_user) "\
>
> ---
> base-commit: 77930e0447e0b37a129db0e13c6c6f5e60a3019e
> change-id: 20250612-glibc_zimop-480880581b2b
> --
> - debug
>