Re: [PATCH] target/riscv/cpu.c: check priv_ver before auto-enable zca/zcd/zcf

[LIU Zhiwei]

On 2023/7/17 23:41, Daniel Henrique Barboza wrote:
> Commit bd30559568 made changes in how we're checking and disabling
> extensions based on env->priv_ver. One of the changes was to move the
> extension disablement code to the end of realize(), being able to
> disable extensions after we've auto-enabled some of them.
>
> An unfortunate side effect of this change started to happen with CPUs
> that has an older priv version, like sifive-u54. Starting on commit
> 2288a5ce43e5 we're auto-enabling zca, zcd and zcf if RVC is enabled,
> but these extensions are priv version 1.12.0. When running a cpu that
> has an older priv ver (like sifive-u54) the user is spammed with
> warnings like these:
>
> qemu-system-riscv64: warning: disabling zca extension for hart 
> 0x0000000000000000 because privilege spec version does not match
> qemu-system-riscv64: warning: disabling zcd extension for hart 
> 0x0000000000000000 because privilege spec version does not match
>
> The warnings are part of the code that disables the extension, but in this
> case we're throwing user warnings for stuff that we enabled on our own,
> without user intervention. Users are left wondering what they did wrong.
>
> A quick 8.1 fix for this nuisance is to check the CPU priv spec before
> auto-enabling zca/zcd/zcf. A more appropriate fix will include a more
> robust framework that will account for both priv_ver and user choice
> when auto-enabling/disabling extensions, but for 8.1 we'll make it do
> with this simple check.
>
> It's also worth noticing that this is the only case where we're
> auto-enabling extensions based on a criteria (in this case RVC) that
> doesn't match the priv spec of the extensions we're enabling. There's no
> need for more 8.1 band-aids.
>
> Cc: Conor Dooley <conor@kernel.org>
> Fixes: 2288a5ce43e5 ("target/riscv: add cfg properties for Zc* extension")
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> ---
>   target/riscv/cpu.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index 9339c0241d..6b93b04453 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -1225,7 +1225,8 @@ void riscv_cpu_validate_set_extensions(RISCVCPU *cpu, 
> Error **errp)
>           }
>       }
>   
> -    if (riscv_has_ext(env, RVC)) {
> +    /* zca, zcd and zcf has a PRIV 1.12.0 restriction */

I think the Zca/zcd/zcf doesn't have much relationship with the 
privilege specification. The privilege specification doesn't define any
CSR or rules that Zca/zcd/zcf depend on. Maybe I missed something.  Does 
anyone  know why we should check PRIV_VERSION_1_12_0 for zca/zcf/zcd?

I think we should remove the check for priv_ver for many user mode 
extensions. We should set the checking privilege specification version 
for these extensions to PRIV_VERSION_1_10_0.

Zhiwei

> +    if (riscv_has_ext(env, RVC) && env->priv_ver >= PRIV_VERSION_1_12_0) {
>           cpu->cfg.ext_zca = true;
>           if (riscv_has_ext(env, RVF) && env->misa_mxl_max == MXL_RV32) {
>               cpu->cfg.ext_zcf = true;