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

[Vivek Gautam]

On Mon, Jan 21, 2019 at 7:55 PM Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
>
> On Mon, 21 Jan 2019 at 14:56, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> >
> > On 21/01/2019 13:36, Ard Biesheuvel wrote:
> > > 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?
> >
> > Not quite - in general they need INC-ONC attributes in case there
> > actually is something in the architectural outer-cacheable domain.
>
> But is it a problem to use INC-ONC attributes for the SMMU PTW on this
> chip? AIUI, the reason for the SMMU changes is to avoid the
> performance hit of snooping, which is more expensive than cache
> maintenance of SMMU page tables. So are you saying the by-VA cache
> maintenance is not relayed to this system cache, resulting in page
> table updates to be invisible to masters using INC-ONC attributes?

The reason for this SMMU changes is that the non-coherent devices
can't access the inner caches at all. But they have a way to allocate
and lookup in system cache.

CPU will by default make use of system cache when the inner-cacheable
and outer-cacheable memory attribute is set.

So for SMMU page tables to be visible to PTW,
-- For IO coherent clients, the CPU cache maintenance operations are not
required for buffers marked Normal Cached to achieve a coherent view of
memory. However, client-specific cache maintenance may still be
required for devices
with local caches (for example, compute DSP local L1 or L2).
-- For non-IO coherent clients, the CPU cache maintenance operations (cleans
and/or invalidates) are required at buffer handoff points for buffers marked as
Normal Cached in any CPU page table in order to observe the latest updates.


Regards
Vivek

>
> > The
> > case of the outer cacheablility being not that but a hint to control
> > non-CPU traffic through some not-quite-transparent cache behind the PoC
> > definitely stays wrapped up in qcom-specific magic ;)
> >
>
> I'm not surprised ...



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