Messages in this thread | | | Date | Wed, 30 Jun 2021 15:37:59 +0530 | From | Sai Prakash Ranjan <> | Subject | Re: [PATCH 2/3] iommu/io-pgtable-arm: Add IOMMU_LLC page protection flag |
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Hi Will,
On 2021-03-25 23:03, Will Deacon wrote: > 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@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. > > Do we have a solution to the mismatched virtual alias? >
Sorry for the long delay on this thread.
For mismatched virtual alias question, wasn't this already discussed in stretch when initial support for system cache [1] (which was reverted by you) was added?
Excerpt from there,
"As seen in downstream kernels there are few non-coherent devices which would not want to allocate in system cache, and therefore would want Inner/Outer non-cached memory. So, we may want to either override the attributes per-device, or as you suggested we may want to introduce another memory type 'sys-cached' that can be added with its separate infra."
As for DMA API usage, we do not have any upstream users (video will be one if they decide to upstream that).
[1] https://patchwork.kernel.org/project/linux-arm-msm/patch/20180615105329.26800-1-vivek.gautam@codeaurora.org/
Thanks, Sai
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