Re: [Xen-devel] [PATCH v2] drm/xen-front: Make shmem backed display buffer coherent

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

On 1/18/19 1:43 PM, Julien Grall wrote:
> (+ 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?
>
It takes quite some time to collect this (because many components are 
involved in the
use-case), but for now the pages in the guest domain are:
!PTE_RDONLY + PTE_PXN + PTE_SHARED + PTE_AF + PTE_UXN + 
PTE_ATTRINDX(MT_NORMAL)

> Cheers,
>
>>
>> Thank you,
>> Oleksandr
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>
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