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

Clément Léger
2025-03-14T12:23:28Z

On 14/03/2025 13:02, Andrew Jones wrote:
> On Fri, Mar 14, 2025 at 12:33:55PM +0100, Clément Léger wrote:
>>
>>
>> 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.
>
> At least the comment should drop the word 'Reset' and if there's a chance
> that not all harts having the same value then we should call get on all
> of them. (We'll probably want SBI FWFT functions which operate on
> hartmasks eventually.)

Ok, then I can pass a cpu_mask as well so that caller just have to pass
online_cpus() if they want it on all cpus.

>
>>
>>>
>>>> + 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 ?).
>
> How about offlining all harts which don't have the desired state, along
> with complaining loudly to the boot log.
>
> Thanks,
> drew