Re: [Qemu-devel] [PATCH v4] Improve the alignment check infrastructure

[Richard Henderson <rth@twiddle.net>]

On 06/23/2016 11:16 AM, Sergey Sorokin wrote:
> +#if defined(CONFIG_SOFTMMU)
> +/**
> + * get_alignment_bits
> + * @memop: TCGMemOp value
> + *
> + * Extract the alignment size from the memop.
> + *
> + * Returns: 0 in case of byte access (which is always aligned);
> + *          positive value - number of alignment bits;
> + *          negative value if unaligned access enabled
> + *          and this is not a byte access.
> + */
> +static inline int get_alignment_bits(TCGMemOp memop)
> +{
> +    int a = memop & MO_AMASK;
> +    int s = memop & MO_SIZE;
> +
> +    if (a == MO_UNALN) {
> +        /* Negative value if unaligned access enabled,
> +         * or zero value in case of byte access.
> +         */
> +        return -s;
> +    } else if (a == MO_ALIGN) {
> +        tcg_debug_assert((TLB_FLAGS_MASK & ((1 << s) - 1)) == 0);
> +        /* A natural alignment: return a number of access size bits */
> +        return s;
> +    } else {
> +        /* Specific alignment size. It must be equal or greater
> +         * than the access size.
> +         */
> +        a >>= MO_ASHIFT;
> +        tcg_debug_assert(a >= s);
> +        tcg_debug_assert((TLB_FLAGS_MASK & ((1 << a) - 1)) == 0);
> +        return a;
> +    }
> +}
> +#endif  /* CONFIG_SOFTMMU */

While it's true that usermode doesn't support alignment checks at all (either 
direction, I'd prefer to leave the function available and isolate the one 
assert that caused your build problem.  E.g.

static inline int get_alignment_bits(TCGMemOp memop)
{
     int a = memop & MO_AMASK;
     int s = memop & MO_SIZE;
     int r;

     ...
     } else if (a == MO_ALIGN) {
         /* A natural alignment: return a number of access size bits */
         r = s;
     } else {
         /* Specific alignment size. It must be equal or greater
          * than the access size.
          */
         r = a >> MO_ASHIFT;
         tcg_debug_assert(r >= s);
     }
#ifdef CONFIG_SOFTMMU
     /* Make sure requested alignment doesn't overlap TLB flags.  */
     tcg_debug_assert((TLB_FLAGS_MASK & ((1 << r) - 1)) == 0);
#endif
     return r;
}


I'll give this a test with some target-sparc changes where this will be useful.


r~