Re: [PATCH] riscv: fix multi-letter extension compare

[Jessica Clarke]

On 6 Oct 2022, at 08:02, Nikita Shubin <nikita.shubin@xxxxxxxxxxx> wrote:
> 
> From: Nikita Shubin <n.shubin@xxxxxxxxx>
> 
> Increment ext pointer to match letters after 'z' character, so it points
> to actual extension name.
> 
> Fixes: 4905ec2fb7e6 ("RISC-V: Add sscofpmf extension support")
> Signed-off-by: Nikita Shubin <n.shubin@xxxxxxxxx>
> ---
> Hello Atish,
> 
> Sorry to blame you, but i double checked this and i don't see a way 
> how extensions from device tree could be successfully matched with 
> encoded ones without incrementing the ext pointer or prepending 
> the encoded extensions with 'z' letter.
> 
> I have no idea how this could slip through review - am i missing something ?
> ---
> arch/riscv/kernel/cpufeature.c | 1 +
> 1 file changed, 1 insertion(+)
> 
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 3b5583db9d80..031546052dc0 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -199,6 +199,7 @@ void __init riscv_fill_hwcap(void)
> 				this_hwcap |= isa2hwcap[(unsigned char)(*ext)];
> 				set_bit(*ext - 'a', this_isa);
> 			} else {
> +				ext++;
> 				SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
> 				SET_ISA_EXT_MAP("svpbmt", RISCV_ISA_EXT_SVPBMT);
> 				SET_ISA_EXT_MAP("zicbom", RISCV_ISA_EXT_ZICBOM);

This makes no sense. The single-letter prefix that’s part of the actual
extension name is already in the string there, whether s or z. Your
code means (a) you’d have to double up the prefix in the device tree
(which is obviously wrong) (b) you could use s or z interchangeably as
the first character (e.g. you’d make zzicbom and szicbom parsed as
zicbom).

Are you actually seeing a bug? Did you test this at all? This looks
like you misunderstood code and wrote an untested patch for a bug you
thought you saw.

Jess

> -- 
> 2.35.1
> 
> 
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