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

[Will Deacon]

On Tue, Mar 09, 2021 at 12:10:44PM +0530, Sai Prakash Ranjan wrote:
> 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@xxxxxxxxxx> 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.

Do we have a solution to the mismatched virtual alias?

Will