[PATCH] target/riscv: Don't start user-mode with VILL

[PATCH] target/riscv: Don't start user-mode with VILL

[Palmer Dabbelt]

This is still under discussion in the psABI, but it's looking like we're
going to forbid VILL in userspace in order to maintain compatibility
with binaries that don't expect implementations to trap whole register
moves under VILL (as in QEMU before 4eff52cd46 ("target/riscv: Add vill
check for whole vector register move instructions"), for example).

Fixes: f8c1f36a2e ("target/riscv: Set vtype.vill on CPU reset")
Link: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/454
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
---
 target/riscv/cpu.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index f219f0c3b5..d19a44de99 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -1022,7 +1022,11 @@ static void riscv_cpu_reset_hold(Object *obj, ResetType type)
     cs->exception_index = RISCV_EXCP_NONE;
     env->load_res = -1;
     set_default_nan_mode(1, &env->fp_status);
+#ifdef CONFIG_USER_ONLY
+    env->vill = false;
+#else
     env->vill = true;
+#endif
 
 #ifndef CONFIG_USER_ONLY
     if (cpu->cfg.debug) {
-- 
2.45.2

Re: [PATCH] target/riscv: Don't start user-mode with VILL

[Richard Henderson]

On 11/21/24 18:32, Palmer Dabbelt wrote:
> This is still under discussion in the psABI, but it's looking like we're
> going to forbid VILL in userspace in order to maintain compatibility
> with binaries that don't expect implementations to trap whole register
> moves under VILL (as in QEMU before 4eff52cd46 ("target/riscv: Add vill
> check for whole vector register move instructions"), for example).
> 
> Fixes: f8c1f36a2e ("target/riscv: Set vtype.vill on CPU reset")
> Link: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/454
> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> ---
>   target/riscv/cpu.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index f219f0c3b5..d19a44de99 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -1022,7 +1022,11 @@ static void riscv_cpu_reset_hold(Object *obj, ResetType type)
>       cs->exception_index = RISCV_EXCP_NONE;
>       env->load_res = -1;
>       set_default_nan_mode(1, &env->fp_status);
> +#ifdef CONFIG_USER_ONLY
> +    env->vill = false;
> +#else
>       env->vill = true;
> +#endif
>   
>   #ifndef CONFIG_USER_ONLY
>       if (cpu->cfg.debug) {

This suggests that vtype_check() in tcg/riscv/tcg-target.c.inc needs modification.  If the 
probe fails, we need to set vtype back to any valid value (e.g. 0).

Also, may I suggest stronger guidelines about which VSEW are required to be supported in 
various configurations.  Right now, it appears as if the static compilers simply assume 
all combinations of SEW/LMUL work.


r~