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

[Rob Clark]

On Wed, Feb 3, 2021 at 2:14 PM Rob Clark <robdclark@xxxxxxxxx> wrote:
>
> On Wed, Feb 3, 2021 at 1:46 PM Will Deacon <will@xxxxxxxxxx> 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?
>
> Currently userspace asks for everything WC, so pgprot_writecombine()
>
> The kernel doesn't enforce this, but so far provides no UAPI to do
> anything useful with non-coherent cached mappings (although there is
> interest to support this)
>

btw, I'm looking at a benchmark (gl_driver2_off) where (after some
other in-flight optimizations land) we end up bottlenecked on writing
to WC cmdstream buffers.  I assume in the current state, WC goes all
the way to main memory rather than just to system cache?

BR,
-R

> BR,
> -R
>
> > 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