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

[Oleksandr Andrushchenko]

On 12/19/18 4:10 PM, Gerd Hoffmann wrote:
>    Hi,
>
> > > 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?
> I would try without that, and maybe add a runtime option (module
> parameter) later if it turns out some hardware actually needs that.
> Devices which can do 32bit DMA only become less and less common these
> days.
Good point, will remove then
> > > > > > +ÂÂÂ if (!dma_map_sg(dev->dev, xen_obj->sgt->sgl, xen_obj->sgt->nents,
> > > > > > +ÂÂÂÂÂÂÂÂÂÂÂ DMA_BIDIRECTIONAL)) {
> > > > > Are you using the DMA streaming API as a way to flush the caches?
> > > > Yes
> > > > > 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...
> Oh, arm.  Maybe ask on a arm list then.  I know on arm you have to care
> about caching a lot more, but that also is where my knowledge ends ...
>
> Using dma_map_sg for cache flushing looks like a sledge hammer approach
> to me.
It is. This is why I am so unsure this is way to go
>    But maybe it is needed to make xen flush the caches (xen guests
> have their own dma mapping implementation, right?  Or is this different
> on arm than on x86?).
I'll try to figure out
> cheers,
>    Gerd
Thank you,
Oleksandr