Re: [PATCH 0/3] iommu/arm-smmu: Add support to use Last level cache

[Robin Murphy <robin.murphy@arm.com>]

On 21/01/2019 10:50, Ard Biesheuvel wrote:
> On Mon, 21 Jan 2019 at 11:17, Vivek Gautam <vivek.gautam@codeaurora.org> wrote:
>>
>> Hi,
>>
>>
>> On Mon, Jan 21, 2019 at 12:56 PM Ard Biesheuvel
>> <ard.biesheuvel@linaro.org> wrote:
>>>
>>> On Mon, 21 Jan 2019 at 06:54, Vivek Gautam <vivek.gautam@codeaurora.org> wrote:
>>>>
>>>> Qualcomm SoCs have an additional level of cache called as
>>>> System cache, aka. Last level cache (LLC). This cache sits right
>>>> before the DDR, and is tightly coupled with the memory controller.
>>>> The clients using this cache request their slices from this
>>>> system cache, make it active, and can then start using it.
>>>> For these clients with smmu, to start using the system cache for
>>>> buffers and, related page tables [1], memory attributes need to be
>>>> set accordingly. This series add the required support.
>>>>
>>>
>>> Does this actually improve performance on reads from a device? The
>>> non-cache coherent DMA routines perform an unconditional D-cache
>>> invalidate by VA to the PoC before reading from the buffers filled by
>>> the device, and I would expect the PoC to be defined as lying beyond
>>> the LLC to still guarantee the architected behavior.
>>
>> We have seen performance improvements when running Manhattan
>> GFXBench benchmarks.
>>
> 
> Ah ok, that makes sense, since in that case, the data flow is mostly
> to the device, not from the device.
> 
>> As for the PoC, from my knowledge on sdm845 the system cache, aka
>> Last level cache (LLC) lies beyond the point of coherency.
>> Non-cache coherent buffers will not be cached to system cache also, and
>> no additional software cache maintenance ops are required for system cache.
>> Pratik can add more if I am missing something.
>>
>> To take care of the memory attributes from DMA APIs side, we can add a
>> DMA_ATTR definition to take care of any dma non-coherent APIs calls.
>>
> 
> So does the device use the correct inner non-cacheable, outer
> writeback cacheable attributes if the SMMU is in pass-through?
> 
> We have been looking into another use case where the fact that the
> SMMU overrides memory attributes is causing issues (WC mappings used
> by the radeon and amdgpu driver). So if the SMMU would honour the
> existing attributes, would you still need the SMMU changes?

Even if we could force a stage 2 mapping with the weakest pagetable 
attributes (such that combining would work), there would still need to 
be a way to set the TCR attributes appropriately if this behaviour is 
wanted for the SMMU's own table walks as well.

Robin.