Messages in this thread | | | Subject | Re: [Xen-devel] [PATCH v2] drm/xen-front: Make shmem backed display buffer coherent | From | Julien Grall <> | Date | Fri, 18 Jan 2019 11:43:32 +0000 |
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(+ Stefano)
Hi,
Sorry for jumping late in the conversation.
On 18/01/2019 09:40, Oleksandr Andrushchenko wrote: > On 1/17/19 11:18 AM, Christoph Hellwig wrote: >> On Wed, Jan 16, 2019 at 06:43:29AM +0000, Oleksandr Andrushchenko wrote: >>>> This whole issue keeps getting more and more confusing. >>> Well, I don't really do DMA here, but instead the buffers in >>> question are shared with other Xen domain, so effectively it >>> could be thought of some sort of DMA here, where the "device" is >>> that remote domain. If the buffers are not flushed then the >>> remote part sees some inconsistency which in my case results >>> in artifacts on screen while displaying the buffers. >>> When buffers are allocated via DMA API then there are no artifacts; >>> if buffers are allocated with shmem + DMA mapping then there are no >>> artifacts as well. >>> The only offending use-case is when I use shmem backed buffers, >>> but do not flush them >> The right answer would be to implement cache maintainance hooks for >> this case in the Xen arch code. These would basically look the same >> as the low-level cache maintainance used by the DMA ops, but without >> going through the DMA mapping layer, in fact they should probably >> reuse the same low-level assembly routines. >> >> I don't think this is the first usage of such Xen buffer sharing, so >> what do the other users do? > I'll have to get even deeper into it. Initially I > looked at the code, but didn't find anything useful. > Or maybe I have just overlooked obvious things there From Xen on Arm ABI:
"All memory which is shared with other entities in the system (including the hypervisor and other guests) must reside in memory which is mapped as Normal Inner Write-Back Outer Write-Back Inner-Shareable. This applies to: - hypercall arguments passed via a pointer to guest memory. - memory shared via the grant table mechanism (including PV I/O rings etc). - memory shared with the hypervisor (struct shared_info, struct vcpu_info, the grant table, etc). "
So you should not need any cache maintenance here. Can you provide more details on the memory attribute you use for memory shared in both the backend and frontend?
Cheers,
> > Thank you, > Oleksandr > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xenproject.org > https://lists.xenproject.org/mailman/listinfo/xen-devel >
-- Julien Grall
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