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Re: [PATCH] target/riscv: fix vs() to return proper error code

From: Richard Henderson
Subject: Re: [PATCH] target/riscv: fix vs() to return proper error code
Date: Tue, 23 Feb 2021 10:46:26 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 2/22/21 10:59 PM, frank.chang@sifive.com wrote:
> From: Frank Chang <frank.chang@sifive.com>
> vs() should return -RISCV_EXCP_ILLEGAL_INST instead of -1 if rvv feature
> is not enabled.
> If -1 is returned, exception will be raised and cs->exception_index will
> be set to the negative return value. The exception will then be treated
> as an instruction access fault instead of illegal instruction fault.

It does seem an unfortunate interface; -1 seems so tempting, but does not by
itself mean anything.

I wonder if we should dispense with the whole "negative number" thing and
simply return an exception value.  Then for bonus points put all of the
RISCV_EXCP_* values in an enumeration, and return that type from these
functions so that it's perfectly clear what the interface really is.

That said,

> @@ -54,7 +54,7 @@ static int vs(CPURISCVState *env, int csrno)
>      if (env->misa & RVV) {
>          return 0;
>      }
> -    return -1;

this fixes the immediate bug, so
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


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