Re: [PATCH v3 02/17] riscv: sbi: add FWFT extension interface

Andrew Jones
2025-03-13T12:39:17Z

On Mon, Mar 10, 2025 at 04:12:09PM +0100, Clément Léger wrote:
> This SBI extensions enables supervisor mode to control feature that are
> under M-mode control (For instance, Svadu menvcfg ADUE bit, Ssdbltrp
> DTE, etc).
>
> Signed-off-by: Clément Léger <cleger@rivosinc.com>
> ---
> arch/riscv/include/asm/sbi.h | 5 ++
> arch/riscv/kernel/sbi.c | 97 ++++++++++++++++++++++++++++++++++++
> 2 files changed, 102 insertions(+)
>
> diff --git a/arch/riscv/include/asm/sbi.h b/arch/riscv/include/asm/sbi.h
> index bb077d0c912f..fc87c609c11a 100644
> --- a/arch/riscv/include/asm/sbi.h
> +++ b/arch/riscv/include/asm/sbi.h
> @@ -503,6 +503,11 @@ int sbi_remote_hfence_vvma_asid(const struct cpumask *cpu_mask,
> unsigned long asid);
> long sbi_probe_extension(int ext);
>
> +int sbi_fwft_all_cpus_set(u32 feature, unsigned long value, unsigned long flags,
> + bool revert_on_failure);
> +int sbi_fwft_get(u32 feature, unsigned long *value);
> +int sbi_fwft_set(u32 feature, unsigned long value, unsigned long flags);
> +
> /* Check if current SBI specification version is 0.1 or not */
> static inline int sbi_spec_is_0_1(void)
> {
> diff --git a/arch/riscv/kernel/sbi.c b/arch/riscv/kernel/sbi.c
> index 1989b8cade1b..256910db1307 100644
> --- a/arch/riscv/kernel/sbi.c
> +++ b/arch/riscv/kernel/sbi.c
> @@ -299,6 +299,103 @@ static int __sbi_rfence_v02(int fid, const struct cpumask *cpu_mask,
> return 0;
> }
>
> +int sbi_fwft_get(u32 feature, unsigned long *value)
> +{
> + return -EOPNOTSUPP;
> +}
> +
> +/**
> + * sbi_fwft_set() - Set a feature on all online cpus

copy+paste of description from sbi_fwft_all_cpus_set(). This function
only sets the feature on the calling hart.

> + * @feature: The feature to be set
> + * @value: The feature value to be set
> + * @flags: FWFT feature set flags
> + *
> + * Return: 0 on success, appropriate linux error code otherwise.
> + */
> +int sbi_fwft_set(u32 feature, unsigned long value, unsigned long flags)
> +{
> + return -EOPNOTSUPP;
> +}
> +
> +struct fwft_set_req {
> + u32 feature;
> + unsigned long value;
> + unsigned long flags;
> + cpumask_t mask;
> +};
> +
> +static void cpu_sbi_fwft_set(void *arg)
> +{
> + struct fwft_set_req *req = arg;
> +
> + if (sbi_fwft_set(req->feature, req->value, req->flags))
> + cpumask_clear_cpu(smp_processor_id(), &req->mask);
> +}
> +
> +static int sbi_fwft_feature_local_set(u32 feature, unsigned long value,
> + unsigned long flags,
> + bool revert_on_fail)
> +{
> + int ret;
> + unsigned long prev_value;
> + cpumask_t tmp;
> + struct fwft_set_req req = {
> + .feature = feature,
> + .value = value,
> + .flags = flags,
> + };
> +
> + cpumask_copy(&req.mask, cpu_online_mask);
> +
> + /* We can not revert if features are locked */
> + if (revert_on_fail && flags & SBI_FWFT_SET_FLAG_LOCK)

Should use () around the flags &. I thought checkpatch complained about
that?

> + return -EINVAL;
> +
> + /* Reset value is the same for all cpus, read it once. */

How do we know we're reading the reset value? sbi_fwft_all_cpus_set() may
be called multiple times on the same feature. And harts may have had
sbi_fwft_set() called on them independently. I think we should drop the
whole prev_value optimization.

> + ret = sbi_fwft_get(feature, &prev_value);
> + if (ret)
> + return ret;
> +
> + /* Feature might already be set to the value we want */
> + if (prev_value == value)
> + return 0;
> +
> + on_each_cpu_mask(&req.mask, cpu_sbi_fwft_set, &req, 1);
> + if (cpumask_equal(&req.mask, cpu_online_mask))
> + return 0;
> +
> + pr_err("Failed to set feature %x for all online cpus, reverting\n",
> + feature);

nit: I'd let the above line stick out. We have 100 chars.

> +
> + req.value = prev_value;
> + cpumask_copy(&tmp, &req.mask);
> + on_each_cpu_mask(&req.mask, cpu_sbi_fwft_set, &req, 1);
> + if (cpumask_equal(&req.mask, &tmp))
> + return 0;

I'm not sure we want the revert_on_fail support either. What happens when
the revert fails and we return -EINVAL below? Also returning zero when
revert succeeds means the caller won't know if we successfully set what
we wanted or just successfully reverted.

> +
> + return -EINVAL;
> +}
> +
> +/**
> + * sbi_fwft_all_cpus_set() - Set a feature on all online cpus
> + * @feature: The feature to be set
> + * @value: The feature value to be set
> + * @flags: FWFT feature set flags
> + * @revert_on_fail: true if feature value should be restored to it's orignal

its original

> + * value on failure.

Line 'value' up under 'true'

> + *
> + * Return: 0 on success, appropriate linux error code otherwise.
> + */
> +int sbi_fwft_all_cpus_set(u32 feature, unsigned long value, unsigned long flags,
> + bool revert_on_fail)
> +{
> + if (feature & SBI_FWFT_GLOBAL_FEATURE_BIT)
> + return sbi_fwft_set(feature, value, flags);
> +
> + return sbi_fwft_feature_local_set(feature, value, flags,
> + revert_on_fail);
> +}
> +
> /**
> * sbi_set_timer() - Program the timer for next timer event.
> * @stime_value: The value after which next timer event should fire.
> --
> 2.47.2

Thanks,
drew