Re: [PATCH 2/3] iommu/io-pgtable-arm: Add IOMMU_LLC page protection flag

[Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>]

Hi,

On 2021-02-05 17:38, Sai Prakash Ranjan wrote:
> On 2021-02-04 03:16, Will Deacon wrote:
>> On Tue, Feb 02, 2021 at 11:56:27AM +0530, Sai Prakash Ranjan wrote:
>>> On 2021-02-01 23:50, Jordan Crouse wrote:
>>> > On Mon, Feb 01, 2021 at 08:20:44AM -0800, Rob Clark wrote:
>>> > > On Mon, Feb 1, 2021 at 3:16 AM Will Deacon <will@kernel.org> wrote:
>>> > > > On Fri, Jan 29, 2021 at 03:12:59PM +0530, Sai Prakash Ranjan wrote:
>>> > > > > On 2021-01-29 14:35, Will Deacon wrote:
>>> > > > > > On Mon, Jan 11, 2021 at 07:45:04PM +0530, Sai Prakash Ranjan wrote:
>>> > > > > > > +#define IOMMU_LLC        (1 << 6)
>>> > > > > >
>>> > > > > > On reflection, I'm a bit worried about exposing this because I think it
>>> > > > > > will
>>> > > > > > introduce a mismatched virtual alias with the CPU (we don't even have a
>>> > > > > > MAIR
>>> > > > > > set up for this memory type). Now, we also have that issue for the PTW,
>>> > > > > > but
>>> > > > > > since we always use cache maintenance (i.e. the streaming API) for
>>> > > > > > publishing the page-tables to a non-coheren walker, it works out.
>>> > > > > > However,
>>> > > > > > if somebody expects IOMMU_LLC to be coherent with a DMA API coherent
>>> > > > > > allocation, then they're potentially in for a nasty surprise due to the
>>> > > > > > mismatched outer-cacheability attributes.
>>> > > > > >
>>> > > > >
>>> > > > > Can't we add the syscached memory type similar to what is done on android?
>>> > > >
>>> > > > Maybe. How does the GPU driver map these things on the CPU side?
>>> > >
>>> > > Currently we use writecombine mappings for everything, although there
>>> > > are some cases that we'd like to use cached (but have not merged
>>> > > patches that would give userspace a way to flush/invalidate)
>>> > >
>>> >
>>> > LLC/system cache doesn't have a relationship with the CPU cache.  Its
>>> > just a
>>> > little accelerator that sits on the connection from the GPU to DDR and
>>> > caches
>>> > accesses. The hint that Sai is suggesting is used to mark the buffers as
>>> > 'no-write-allocate' to prevent GPU write operations from being cached in
>>> > the LLC
>>> > which a) isn't interesting and b) takes up cache space for read
>>> > operations.
>>> >
>>> > Its easiest to think of the LLC as a bonus accelerator that has no cost
>>> > for
>>> > us to use outside of the unfortunate per buffer hint.
>>> >
>>> > We do have to worry about the CPU cache w.r.t I/O coherency (which is a
>>> > different hint) and in that case we have all of concerns that Will
>>> > identified.
>>> >
>>> 
>>> For mismatched outer cacheability attributes which Will mentioned, I 
>>> was
>>> referring to [1] in android kernel.
>> 
>> I've lost track of the conversation here :/
>> 
>> When the GPU has a buffer mapped with IOMMU_LLC, is the buffer also 
>> mapped
>> into the CPU and with what attributes? Rob said "writecombine for
>> everything" -- does that mean ioremap_wc() / MEMREMAP_WC?
>> 
> 
> Rob answered this.
> 
>> Finally, we need to be careful when we use the word "hint" as 
>> "allocation
>> hint" has a specific meaning in the architecture, and if we only 
>> mismatch on
>> those then we're actually ok. But I think IOMMU_LLC is more than just 
>> a
>> hint, since it actually drives eviction policy (i.e. it enables 
>> writeback).
>> 
>> Sorry for the pedantry, but I just want to make sure we're all talking
>> about the same things!
>> 
> 
> Sorry for the confusion which probably was caused by my mentioning of
> android, NWA(no write allocate) is an allocation hint which we can 
> ignore
> for now as it is not introduced yet in upstream.
> 

Any chance of taking this forward? We do not want to miss out on small 
fps
gain when the product gets released.

Thanks,
Sai
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