[PATCH v3 4/8] RISC-V: Check Zicclsm to set unaligned access speed

[Clément Léger]

On 27/06/2024 23:20, Charlie Jenkins wrote:
> On Wed, Jun 26, 2024 at 03:39:14PM +0100, Conor Dooley wrote:
>> On Mon, Jun 24, 2024 at 08:49:57PM -0400, Jesse Taube wrote:
>>> Check for Zicclsm before checking for unaligned access speed. This will
>>> greatly reduce the boot up time as finding the access speed is no longer
>>> necessary.
>>>
>>> Signed-off-by: Jesse Taube <jesse at rivosinc.com>
>>> ---
>>> V2 -> V3:
>>>  - New patch split from previous patch
>>> ---
>>>  arch/riscv/kernel/unaligned_access_speed.c | 26 ++++++++++++++--------
>>>  1 file changed, 17 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/arch/riscv/kernel/unaligned_access_speed.c b/arch/riscv/kernel/unaligned_access_speed.c
>>> index a9a6bcb02acf..329fd289b5c8 100644
>>> --- a/arch/riscv/kernel/unaligned_access_speed.c
>>> +++ b/arch/riscv/kernel/unaligned_access_speed.c
>>> @@ -259,23 +259,31 @@ static int check_unaligned_access_speed_all_cpus(void)
>>>  	kfree(bufs);
>>>  	return 0;
>>>  }
>>> +#else /* CONFIG_RISCV_PROBE_UNALIGNED_ACCESS */
>>> +static int check_unaligned_access_speed_all_cpus(void)
>>> +{
>>> +	return 0;
>>> +}
>>> +#endif
>>>  
>>>  static int check_unaligned_access_all_cpus(void)
>>>  {
>>> -	bool all_cpus_emulated = check_unaligned_access_emulated_all_cpus();
>>> +	bool all_cpus_emulated;
>>> +	int cpu;
>>> +
>>> +	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICCLSM)) {
>>> +		for_each_online_cpu(cpu) {
>>> +			per_cpu(misaligned_access_speed, cpu) = RISCV_HWPROBE_MISALIGNED_FAST;
>>
>> - const: zicclsm
>>   description:
>>     The standard Zicclsm extension for misaligned support for all regular
>>     load and store instructions (including scalar and vector) but not AMOs
>>     or other specialized forms of memory access. Defined in the
>>     RISC-V RVA Profiles Specification. 
>>
>> Doesn't, unfortunately, say anywhere there that they're actually fast :(
> 
> Oh no! That is unfortunate that the ISA does not explicitly call that
> out, but I think that acceptable.
> 
> If a vendor puts Zicclsm in their isa string, they should expect
> software to take advantage of misaligned accesses. FAST is our signal to
> tell software that they should emit misaligned accesses.

AFAIK, Zicclsm is not even an ISA extension, simply a profile
specification which means that only the execution environment which
provides the profile support misaligned accesses (cf
https://lists.riscv.org/g/tech-profiles/message/56). I don't think we
can extrapolate that the misaligned accesses will be fast at all.

Thanks,

Clément

> 
> This allows for a generic kernel, like the one a distro would compile, to
> skip the probing when booting on a system that explicitly called out
> that the hardware supports misaligned accesses.
> 
> - Charlie
> 
>>
>> Thanks,
>> Conor.
>>
>>> +		}
>>> +		return 0;
>>> +	}
>>> +
>>> +	all_cpus_emulated = check_unaligned_access_emulated_all_cpus();
>>>  
>>>  	if (!all_cpus_emulated)
>>>  		return check_unaligned_access_speed_all_cpus();
>>>  
>>>  	return 0;
>>>  }
>>> -#else /* CONFIG_RISCV_PROBE_UNALIGNED_ACCESS */
>>> -static int check_unaligned_access_all_cpus(void)
>>> -{
>>> -	check_unaligned_access_emulated_all_cpus();
>>> -
>>> -	return 0;
>>> -}
>>> -#endif
>>>  
>>>  arch_initcall(check_unaligned_access_all_cpus);
>>> -- 
>>> 2.45.2
>>>
> 
>