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

["Clément Léger" <cleger@rivosinc.com>]

On 13/03/2025 13:39, Andrew Jones wrote:
> 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.

That's actually used for revert_on_failure as well not only the
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.

So that might actually be needed for features that needs to be enabled
on all hart or not enabled at all. If we fail to enable all of them,
them the hart will be in some non coherent state between the harts.
The returned error code though is wrong and I'm not sure we would have a
way to gracefully handle revertion failure (except maybe panicking ?).

Thanks,

Clément

> 
>> +
>> +	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