[PATCH v4 5/7] arm64: Add support for FEAT_{LS64, LS64_V}

[Yicong Yang]

On 2025/9/17 22:20, Catalin Marinas wrote:
> On Wed, Sep 17, 2025 at 11:51:20AM +0800, Yicong Yang wrote:
>> On 2025/9/16 22:56, Catalin Marinas wrote:
>>> On Mon, Sep 15, 2025 at 04:29:25PM +0800, Yicong Yang wrote:
>>>> in my understanding the hwcap only describes the capabilities of the CPU but not
>>>> the whole system. the users should make sure the function works as expected if the
>>>> CPU supports it and they're going to use it. specifically the LS64 is intended for
>>>> device memory only, so the user should take responsibility of using it on supported
>>>> memory.
>>>
>>> We have other cases like MTE where we avoid exposing the HWCAP to user
>>> if we know the memory system does not support MTE, though we intercepted
>>> this early and asked the (micro)architects to tie the CPU ID field to
>>> what the system supports.
>>
>> but we lack the same identification mechanism as CPU for the memory system, so it's just a
>> restriction for the hardware vendor that if certain feature is not supported for the whole
>> system (SoC) then do not advertise it in the CPU's ID field. otherwise i think we're currently
>> doing in the manner that if capability mismatch or cannot work as expected together then a
>> errata/workaround is used to disable the feature or add some workaround on this certain
>> platform.
>>
>> this is also the case for LS64 but a bit more complex, since it involves the completer outside
>> the SoC (the device) and could be a hotplug one (PCIe). from the SoC part we can restrict to
>> advertise the feature only if it's fully supported (what we've already done on our hardware).
> 
> That's good to know. Hopefully other vendors do the same.
> 
> I think the ARM ARM would benefit from a note here that the system
> designers should not advertise this if the interconnect does not support
> it. I can raise this internally.
> 

one complementation, only ld64b/st64b (FEAT_LS64) need this concerns. otherwise
for st64bv/st64bv0 that support return a status, user can check the status result:

  If the target memory location does not support the ST64BV or ST64BV0 instructions,
  then the register specified by <Xs> is set to 0xFFFFFFFF_FFFFFFFF.

>>> Arguably, the use of LD/ST64B* is fairly specialised and won't be used
>>> on the general purpose RAM and by random applications. It needs a device
>>> driver to create the NC/Device mapping and specific programs/libraries
>>> to access it. I'm not sure the LS64 properties are guaranteed by the
>>> device alone or the device together with the interconnect. I suspect the
>>> latter and neither the kernel driver nor user space can tell. In the
>>> best case, you get a fault and realise the system doesn't work as
>>> expected. Worse is the non-atomicity with potentially silent corruption.
>>
>> will be the latter one, both interconnect and the target device need to
>> support it. but I think the driver developer (kernel driver or userspace
>> driver) must have knowledge about the support status, otherwise they
>> should not use it.
> [...]
>> my thoughts is that the driver developer should have known whether their
>> device support it or not if going to use this. the information in the
>> firmware table should be fine for platform devices, but cannot describe
>> information for hotpluggable ones like PCIe endpoint devices which may
>> not be listed in a firmware table.
> 
> There's a risk of such instructions ending up in more generic
> copy_to/from_io implementations but it's not much we can do other than
> not enabling the feature at all.
> 
> So, I think a HWCAP bit is useful but we need (a) clarification that the
> CPUID field won't be set if the system doesn't support it and (b)
> document the Linux bit that it's a per-device capability even if the
> CPU/system supports it (the HWCAP is only a prerequisite to be able to
> use the instructions; the driver can fall back to non-atomic ops, maybe
> with a DGH if it helps performance).
> 

sure. will mention this in Documentation/arch/arm64/elf_hwcaps.rst for HWCAP3_LS64
as well as the commit message.

thanks.