[PATCH V4 16/17] riscv: compat: Add COMPAT Kbuild skeletal support

[Arnd Bergmann]

On Tue, Feb 1, 2022 at 11:26 AM Guo Ren <guoren at kernel.org> wrote:
>
> Hi Arnd & Christoph,
>
> The UXL field controls the value of XLEN for U-mode, termed UXLEN,
> which may differ from the
> value of XLEN for S-mode, termed SXLEN. The encoding of UXL is the
> same as that of the MXL
> field of misa, shown in Table 3.1.
>
> Here is the patch. (We needn't exception helper, because we are in
> S-mode and UXL wouldn't affect.)

Looks good to me, just a few details that could be improved

> -#define compat_elf_check_arch(x) ((x)->e_machine == EM_RISCV)
> +#ifdef CONFIG_COMPAT
> +#define compat_elf_check_arch compat_elf_check_arch
> +extern bool compat_elf_check_arch(Elf32_Ehdr *hdr);
> +#endif

No need for the #ifdef
> +}

> +void compat_mode_detect(void)

__init

> +{
> + unsigned long tmp = csr_read(CSR_STATUS);
> + csr_write(CSR_STATUS, (tmp & ~SR_UXL) | SR_UXL_32);
> +
> + if ((csr_read(CSR_STATUS) & SR_UXL) != SR_UXL_32) {
> + csr_write(CSR_STATUS, tmp);
> + return;
> + }
> +
> + csr_write(CSR_STATUS, tmp);
> + compat_mode_support = true;
> +
> + pr_info("riscv: compat: 32bit U-mode applications support\n");
> +}

I think an entry in /proc/cpuinfo would be more helpful than the pr_info at
boot time. Maybe a follow-up patch though, as there is no obvious place
to put it. On other architectures, you typically have a set of space
separated feature names, but riscv has a single string that describes
the ISA, and this feature is technically the support for a second ISA.

         Arnd