[PATCH v7 5/7] arm64: Add support for FEAT_{LS64, LS64_V}

[Zhou Wang]

On 2025/11/14 17:37, Arnd Bergmann wrote:
> On Fri, Nov 14, 2025, at 10:25, Zhou Wang wrote:
>> On 2025/11/14 0:26, Arnd Bergmann wrote:
>>>
>>> Are you using a particular device, or are you trying to enable
>>> the support in general? If you have a specific device you are
>>> working on, does it use the PASID data or not?
>>
>> Many thanks for your careful explanation! I got the pointer here.
>>
>> We have a real device in our SoC, which supports LS64B/ST64B/ST64BV.
>> For ST64BV, Our device just receives 64B data atomically, not interpret
>> it with PASID data.
> 
> Ok, I see. So I assume this is either a kind of dedicated work queue
> where the IOMMU PASID is set up in advance for the user MMIO area,

Yeah, it is something like you mentioned above. MMIO area is binded with
a work queue, a PASID is set up in advance for this work queue.

> or it is a device that does not do any DMA at all, correct?
> 
> In this case, would the device also work correctly with ST64BV0 if
> the ACCDATA register is fixed to a value of zero and you can only
> use the upper 480 bits? Would it also work if there is an
> unpredictable value in ACCDATA that may match the PASID of another
> device used by the same process?

We do not support ST64BV0, so above case will not happen. I think ST64BV0
will trigger a illegal instruction exception in our system.

Best,
Zhou

> 
>          Arnd
> .