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

[Will Deacon <will@kernel.org>]

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?

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!

Cheers,

Will