Re: [PATCH v2 1/2] target/riscv: fix vector register address calculation in strided LD/ST

[Eric Biggers <ebiggers@kernel.org>]

On Sat, Aug 16, 2025 at 03:55:40AM +0800, Chao Liu wrote:
> This patch fixes a critical bug in the RISC-V vector instruction
> translation that caused incorrect data handling in strided load
> operations (e.g., vlsseg8e32).
> 
> Problem Description:
> 
> The `get_log2` function in `trans_rvv.c.inc` returned a value 1 higher
> than the actual log2 value. For example, get_log2(4) incorrectly
> returned 3 instead of 2.
> 
> This led to erroneous vector register offset calculations, resulting in
> data overlap where bytes 32-47 were incorrectly copied to positions
> 16-31 in ChaCha20 encryption code.
> 
> rvv_test_func:
>     vsetivli    zero, 1, e32, m1, ta, ma
>     li          t0, 64
> 
>     vlsseg8e32.v v0, (a0), t0
>     addi        a0, a0, 32
>     vlsseg8e32.v v8, (a0), t0
> 
>     vssseg8e32.v v0, (a1), t0
>     addi        a1, a1, 32
>     vssseg8e32.v v8, (a1), t0
>     ret
> 
> Analysis:
> 
> The original implementation counted the number of right shifts until
> zero, including the final shift that reduced the value to zero:
> 
> static inline uint32_t get_log2(uint32_t a)
> {
>     uint32_t i = 0;
>     for (; a > 0;) {
>         a >>= 1;
>         i++;
>     }
>     return i; // Returns 3 for a=4 (0b100 → 0b10 → 0b1 → 0b0)
> }
> 
> Fix:
> 
> The corrected function stops shifting when only the highest bit remains
> and handles the special case of a=0:
> 
> static inline uint32_t get_log2(uint32_t a)
> {
>     uint32_t i = 0;
>     if (a == 0) {
>         return i; // Handle edge case
>     }
>     for (; a > 1; a >>= 1) {
>         i++;
>     }
>     return i; // Now returns 2 for a=4
> }
> 
> Fixes: 28c12c1f2f ("Generate strided vector loads/stores with tcg nodes.")
> 
> Signed-off-by: Chao Liu <chao.liu@yeah.net>
> ---
>  target/riscv/insn_trans/trans_rvv.c.inc | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 

Tested-by: Eric Biggers <ebiggers@kernel.org>

But, to get this to apply I had to re-apply the fixed commit (which was
reverted), then resolve a conflict.  You'll need to send out a new
series which applies to the latest master branch.

- Eric