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

[Ard Biesheuvel]

On Mon, 21 Jan 2019 at 14:25, Robin Murphy <robin.murphy@xxxxxxx> wrote:
>
> On 21/01/2019 10:50, Ard Biesheuvel wrote:
> > On Mon, 21 Jan 2019 at 11:17, Vivek Gautam <vivek.gautam@xxxxxxxxxxxxxx> wrote:
> >>
> >> Hi,
> >>
> >>
> >> On Mon, Jan 21, 2019 at 12:56 PM Ard Biesheuvel
> >> <ard.biesheuvel@xxxxxxxxxx> wrote:
> >>>
> >>> On Mon, 21 Jan 2019 at 06:54, Vivek Gautam <vivek.gautam@xxxxxxxxxxxxxx> 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.
>

Isn't that just a matter of implementing support for SMMUs that lack
the 'dma-coherent' attribute?