linux-kernel - Re: [PATCH -fixes] riscv: cpufeature: Do not drop Linux-internal extensions

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Message-ID: <20240718-a5097a4070f457529d2fff91@orel>
Date: Thu, 18 Jul 2024 16:57:07 -0500
From: Andrew Jones <ajones@...tanamicro.com>
To: Samuel Holland <samuel.holland@...ive.com>
Cc: Palmer Dabbelt <palmer@...belt.com>, linux-riscv@...ts.infradead.org, 
	Albert Ou <aou@...s.berkeley.edu>, Andy Chiu <andy.chiu@...ive.com>, 
	Charlie Jenkins <charlie@...osinc.com>, Clément Léger <cleger@...osinc.com>, 
	Conor Dooley <conor@...nel.org>, Evan Green <evan@...osinc.com>, 
	Paul Walmsley <paul.walmsley@...ive.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -fixes] riscv: cpufeature: Do not drop Linux-internal
 extensions

On Thu, Jul 18, 2024 at 02:29:59PM GMT, Samuel Holland wrote:
> The Linux-internal Xlinuxenvcfg ISA extension is omitted from the
> riscv_isa_ext array because it has no DT binding and should not appear
> in /proc/cpuinfo. The logic added in commit 625034abd52a ("riscv: add
> ISA extensions validation callback") assumes all extensions are included
> in riscv_isa_ext, and so riscv_resolve_isa() wrongly drops Xlinuxenvcfg
> from the final ISA string. Instead, accept such Linux-internal ISA
> extensions as if they have no validation callback.

This assumes we'll never need a validation callback for a Linux-internal
ISA extension. We can make that assumption now and change our mind
later, but we could also add Xlinuxenvcfg to riscv_isa_ext now and
modify the places where it matters (just print_isa?). If we add
Xlinuxenvcfg to the array with a NULL name then we could do something
like

 print_isa()
 {
    for (...) {
        ...
        if (!riscv_isa_ext[i].name)
	   continue;
    }
 }

> 
> Fixes: 625034abd52a ("riscv: add ISA extensions validation callback")
> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> ---
> 
>  arch/riscv/kernel/cpufeature.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 0366dc3baf33..dd25677d60de 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -457,28 +457,26 @@ static void __init riscv_resolve_isa(unsigned long *source_isa,
>  		bitmap_copy(prev_resolved_isa, resolved_isa, RISCV_ISA_EXT_MAX);
>  		for_each_set_bit(bit, source_isa, RISCV_ISA_EXT_MAX) {
>  			ext = riscv_get_isa_ext_data(bit);
> -			if (!ext)
> -				continue;
>  
> -			if (ext->validate) {
> +			if (ext && ext->validate) {
>  				ret = ext->validate(ext, resolved_isa);
>  				if (ret == -EPROBE_DEFER) {
>  					loop = true;
>  					continue;
>  				} else if (ret) {
>  					/* Disable the extension entirely */
> -					clear_bit(ext->id, source_isa);
> +					clear_bit(bit, source_isa);
>  					continue;
>  				}
>  			}
>  
> -			set_bit(ext->id, resolved_isa);
> +			set_bit(bit, resolved_isa);
>  			/* No need to keep it in source isa now that it is enabled */
> -			clear_bit(ext->id, source_isa);
> +			clear_bit(bit, source_isa);
>  
>  			/* Single letter extensions get set in hwcap */
> -			if (ext->id < RISCV_ISA_EXT_BASE)
> -				*this_hwcap |= isa2hwcap[ext->id];
> +			if (bit < RISCV_ISA_EXT_BASE)
> +				*this_hwcap |= isa2hwcap[bit];
>  		}
>  	} while (loop && memcmp(prev_resolved_isa, resolved_isa, sizeof(prev_resolved_isa)));
>  }
> -- 
> 2.45.1
>

If we'd rather leave Xlinuxenvcfg out of the array (and generally support
extensions not in the array), then LGTM

Reviewed-by: Andrew Jones <ajones@...tanamicro.com>

Thanks,
drew