Re: [PATCH v2 11/42] accel/tcg: Perform aligned atomic reads in translator_ld

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 3/18/25 14:31, Richard Henderson wrote:
> Perform aligned atomic reads in translator_ld, if possible.
> According to
> 
> https://lore.kernel.org/qemu-devel/20240607101403.1109-1-jim.shu@sifive.com/
> 
> this is required for RISC-V Ziccif.
> 
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>   accel/tcg/translator.c | 42 ++++++++++++++++++++++++++++++++++++++----
>   1 file changed, 38 insertions(+), 4 deletions(-)
> 
> diff --git a/accel/tcg/translator.c b/accel/tcg/translator.c
> index ef1538b4fc..157be33bf6 100644
> --- a/accel/tcg/translator.c
> +++ b/accel/tcg/translator.c
> @@ -265,12 +265,14 @@ static bool translator_ld(CPUArchState *env, DisasContextBase *db,
>   
>       if (likely(((base ^ last) & TARGET_PAGE_MASK) == 0)) {
>           /* Entire read is from the first page. */
> -        memcpy(dest, host + (pc - base), len);
> -        return true;
> +        goto do_read;
>       }
>   
>       if (unlikely(((base ^ pc) & TARGET_PAGE_MASK) == 0)) {
> -        /* Read begins on the first page and extends to the second. */
> +        /*
> +         * Read begins on the first page and extends to the second.
> +         * The unaligned read is never atomic.
> +         */
>           size_t len0 = -(pc | TARGET_PAGE_MASK);
>           memcpy(dest, host + (pc - base), len0);
>           pc += len0;
> @@ -329,7 +331,39 @@ static bool translator_ld(CPUArchState *env, DisasContextBase *db,
>           host = db->host_addr[1];
>       }
>   
> -    memcpy(dest, host + (pc - base), len);
> + do_read:
> +    /*
> +     * Assume aligned reads should be atomic, if possible.
> +     * We're not in a position to jump out with EXCP_ATOMIC.
> +     */
> +    host += pc - base;
> +    switch (len) {
> +    case 2:
> +        if (QEMU_IS_ALIGNED(pc, 2)) {
> +            uint16_t t = qatomic_read((uint16_t *)host);
> +            stw_he_p(dest, t);
> +            return true;
> +        }
> +        break;
> +    case 4:
> +        if (QEMU_IS_ALIGNED(pc, 4)) {
> +            uint32_t t = qatomic_read((uint32_t *)host);
> +            stl_he_p(dest, t);
> +            return true;
> +        }
> +        break;
> +#ifdef CONFIG_ATOMIC64
> +    case 8:
> +        if (QEMU_IS_ALIGNED(pc, 8)) {
> +            uint64_t t = qatomic_read__nocheck((uint64_t *)host);
> +            stq_he_p(dest, t);
> +            return true;
> +        }
> +        break;
> +#endif
> +    }
> +    /* Unaligned or partial read from the second page is not atomic. */
> +    memcpy(dest, host, len);
>       return true;
>   }
>   

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>