Re: [PATCH 3/3] util/cpuinfo-riscv: Use linux __riscv_hwprobe syscall

[Daniel Henrique Barboza]

On 6/27/24 3:03 PM, Richard Henderson wrote:
> With recent linux kernels, there is a syscall to probe for various
> ISA extensions.  These bits were phased in over several kernel
> releases, so we still require checks for symbol availability.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>   meson.build          |  6 ++++++
>   util/cpuinfo-riscv.c | 26 ++++++++++++++++++++++++++
>   2 files changed, 32 insertions(+)
>
> diff --git a/meson.build b/meson.build
> index 97e00d6f59..58afd0125d 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -2837,6 +2837,12 @@ have_cpuid_h = cc.links('''
>     }''')
>   config_host_data.set('CONFIG_CPUID_H', have_cpuid_h)
>   
> +# Don't bother to advertise asm/hwprobe.h for old versions that do
> +# not contain RISCV_HWPROBE_EXT_ZBA.
> +config_host_data.set('CONFIG_ASM_HWPROBE_H',
> +                     cc.has_header_symbol('asm/hwprobe.h',
> +                                          'RISCV_HWPROBE_EXT_ZBA'))
> +

FWIW I looked around Linux and I think we can snapshot hwprobe support by
checking for RISCV_HWPROBE_KEY_IMA_EXT_0 (Linux commit 162e4df137c) if we
ever need hwprobe for exts earlier than ZBA (C and V).

Checking for ZBA is fine for this patch though.


Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>


>   config_host_data.set('CONFIG_AVX2_OPT', get_option('avx2') \
>     .require(have_cpuid_h, error_message: 'cpuid.h not available, cannot enable 
> AVX2') \
>     .require(cc.links('''
> diff --git a/util/cpuinfo-riscv.c b/util/cpuinfo-riscv.c
> index abf799794f..cf59ce83a3 100644
> --- a/util/cpuinfo-riscv.c
> +++ b/util/cpuinfo-riscv.c
> @@ -6,6 +6,11 @@
>   #include "qemu/osdep.h"
>   #include "host/cpuinfo.h"
>   
> +#ifdef CONFIG_ASM_HWPROBE_H
> +#include <asm/hwprobe.h>
> +#include <sys/syscall.h>
> +#endif
> +
>   unsigned cpuinfo;
>   static volatile sig_atomic_t got_sigill;
>   
> @@ -47,6 +52,27 @@ unsigned __attribute__((constructor)) cpuinfo_init(void)
>   #endif
>       left &= ~info;
>   
> +#ifdef CONFIG_ASM_HWPROBE_H
> +    if (left) {
> +        /*
> +         * TODO: glibc 2.40 will introduce <sys/hwprobe.h>, which
> +         * provides __riscv_hwprobe and __riscv_hwprobe_one,
> +         * which is a slightly cleaner interface.
> +         */
> +        struct riscv_hwprobe pair = { .key = RISCV_HWPROBE_KEY_IMA_EXT_0 };
> +        if (syscall(__NR_riscv_hwprobe, &pair, 1, 0, NULL, 0) == 0
> +            && pair.key >= 0) {
> +            info |= pair.value & RISCV_HWPROBE_EXT_ZBA ? CPUINFO_ZBA : 0;
> +            info |= pair.value & RISCV_HWPROBE_EXT_ZBB ? CPUINFO_ZBB : 0;
> +            left &= ~(CPUINFO_ZBA | CPUINFO_ZBB);
> +#ifdef RISCV_HWPROBE_EXT_ZICOND
> +            info |= pair.value & RISCV_HWPROBE_EXT_ZICOND ? CPUINFO_ZICOND : 0;
> +            left &= ~CPUINFO_ZICOND;
> +#endif
> +        }
> +    }
> +#endif /* CONFIG_ASM_HWPROBE_H */
> +
>       if (left) {
>           struct sigaction sa_old, sa_new;
>