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Re: [Xen-devel] [PATCH] drm/xen-front: Make shmem backed display buffer coherent

On 12/19/18 3:14 PM, Gerd Hoffmann wrote:

+    mapping = xen_obj->base.filp->f_mapping;
+    mapping_set_gfp_mask(mapping, GFP_USER | __GFP_DMA32);
Let's see if I understand what you're doing:

Here you say that the pages should be DMA accessible for devices that can
only see 4GB.
Yes, your understanding is correct. As we are a para-virtualized device we
do not have strict requirements for 32-bit DMA. But, via dma-buf export,
the buffer we create can be used by real HW, e.g. one can pass-through
real HW devices into a guest domain and they can import our buffer (yes,
they can be IOMMU backed and other conditions may apply).
So, this is why we are limiting to DMA32 here, just to allow more possible
Sure this actually helps?  It's below 4G in guest physical address
space, so it can be backed by pages which are actually above 4G in host
physical address space ...

Yes, you are right here. This is why I wrote about the IOMMU

and other conditions. E.g. you can have a device which only

expects 32-bit, but thanks to IOMMU it can access pages above

4GiB seamlessly. So, this is why I *hope* that this code *may* help

such devices. Do you think I don't need that and have to remove?

+    if (!dma_map_sg(dev->dev, xen_obj->sgt->sgl, xen_obj->sgt->nents,
+            DMA_BIDIRECTIONAL)) {

Are you using the DMA streaming API as a way to flush the caches?
Does this mean that GFP_USER isn't making the buffer coherent?
No, it didn't help. I had a question [1] if there are any other better way
to achieve the same, but didn't have any response yet. So, I implemented
it via DMA API which helped.
set_pages_array_*() ?

See arch/x86/include/asm/set_memory.h
Well, x86... I am on arm which doesn't define that...

Thank you,


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