Re: [PATCH v3 1/1] target/riscv: fix endless translation loop on big endian systems

[Alistair Francis]

On Tue, Apr 15, 2025 at 6:06 PM Ziqiao Kong <ziqiaokong@gmail.com> wrote:
>
> On big endian systems, pte and updated_pte hold big endian host data
> while pte_pa points to little endian target data. This means the branch
> at cpu_helper.c:1669 will be always satisfied and restart translation,
> causing an endless translation loop.
>
> The correctness of this patch can be deduced by:
>
> old_pte will hold value either from cpu_to_le32/64(pte) or
> cpu_to_le32/64(updated_pte), both of wich is litte endian. After that,
> an in-place conversion by le32/64_to_cpu(old_pte) ensures that old_pte
> now is in native endian, same with pte. Therefore, the endianness of the
> both side of if (old_pte != pte) is correct.
>
> Signed-off-by: Ziqiao Kong <ziqiaokong@gmail.com>

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

> ---
>  target/riscv/cpu_helper.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
> index 6c4391d96b..3233b66e7e 100644
> --- a/target/riscv/cpu_helper.c
> +++ b/target/riscv/cpu_helper.c
> @@ -1662,9 +1662,11 @@ static int get_physical_address(CPURISCVState *env, 
> hwaddr *physical,
>              target_ulong *pte_pa = qemu_map_ram_ptr(mr->ram_block, addr1);
>              target_ulong old_pte;
>              if (riscv_cpu_sxl(env) == MXL_RV32) {
> -                old_pte = qatomic_cmpxchg((uint32_t *)pte_pa, pte, 
> updated_pte);
> +                old_pte = qatomic_cmpxchg((uint32_t *)pte_pa, 
> cpu_to_le32(pte), cpu_to_le32(updated_pte));
> +                old_pte = le32_to_cpu(old_pte);
>              } else {
> -                old_pte = qatomic_cmpxchg(pte_pa, pte, updated_pte);
> +                old_pte = qatomic_cmpxchg(pte_pa, cpu_to_le64(pte), 
> cpu_to_le64(updated_pte));
> +                old_pte = le64_to_cpu(old_pte);
>              }
>              if (old_pte != pte) {
>                  goto restart;
> --
> 2.34.1
>
>