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

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

Dropped in favor of https://lkml.org/lkml/2019/1/29/910

On 1/24/19 5:02 PM, Julien Grall wrote:
>
>
> On 24/01/2019 14:34, Oleksandr Andrushchenko wrote:
>> Hello, Julien!
>
> Hi,
>
>> On 1/22/19 1:44 PM, Julien Grall wrote:
>>>
>>>
>>> On 1/22/19 10:28 AM, Oleksandr Andrushchenko wrote:
>>>> Hello, Julien!
>>>
>>> Hi,
>>>
>>>> On 1/21/19 7:09 PM, Julien Grall wrote:
>>>> Well, I didn't get the attributes of pages at the backend side, but 
>>>> IMO
>>>> those
>>>> do not matter in my use-case (for simplicity I am not using
>>>> zero-copying at
>>>> backend side):
>>>
>>> They are actually important no matter what is your use case. If you
>>> access the same physical page with different attributes, then you are
>>> asking for trouble.
>> So, we have:
>>
>> DomU: frontend side
>> ====================
>> !PTE_RDONLY + PTE_PXN + PTE_SHARED + PTE_AF + PTE_UXN +
>> PTE_ATTRINDX(MT_NORMAL)
>
> I still don't understand how you came up with MT_NORMAL when you seem 
> to confirm...
>
>>
>> DomD: backend side
>> ====================
>> PTE_USER + !PTE_RDONLY + PTE_PXN + PTE_NG + PTE_CONT + PTE_TABLE_BIT +
>> PTE_UXN + PTE_ATTRINDX(MT_NORMAL)
>>
>>   From the above it seems that I don't violate cached/non-cached
>> agreement here
>>>
>>> This is why Xen imposes all the pages shared to have their memory
>>> attributes following some rules. Actually, speaking with Mark R., we
>>> may want to tight a bit more the attributes.
>>>
>>>>
>>>> 1. Frontend device allocates display buffer pages which come from 
>>>> shmem
>>>> and have these attributes:
>>>> !PTE_RDONLY + PTE_PXN + PTE_SHARED + PTE_AF + PTE_UXN +
>>>> PTE_ATTRINDX(MT_NORMAL)
>>>
>>> My knowledge of Xen DRM is inexistent. However, looking at the code in
>>> 5.0-rc2, I don't seem to find the same attributes. For instance
>>> xen_drm_front_gem_prime_vmap and gem_mmap_obj are using
>>> pgprot_writecombine. So it looks like, the mapping will be
>>> non-cacheable on Arm64.
>>>
>>> Can you explain how you came up to these attributes?
>> pgprot_writecombine is PTE_ATTRINDX(MT_NORMAL_NC), so it seems to be
>> applicable here? [1]
>
> ... that MT_NORMAL_NC is used for the frontend pages.
>
> MT_NORMAL_NC is different from MT_NORMAL. The use of the former will 
> result to non-cacheable memory while the latter will result to 
> cacheable memory.
>
> To me, this looks like the exact reason why you see artifact on the 
> display buffer. As the author of this code, can you explain why you 
> decided to use pgprot_writecombine here instead of relying on the 
> default VMA prot?
>
> [...]
>
>>> We actually never required to use cache flush in other PV protocol, so
>>> I still don't understand why the PV DRM should be different here.
>> Well, you are right. But at the same time not flushing the buffer makes
>> troubles,
>> so this is why I am trying to figure out what is wrong here.
>
> The cache flush is likely hiding the real problem rather than solving it.
>
>>>
>>> To me, it looks like that you are either missing some barriers
>> Barriers for the buffer? Not sure what you mean here.
>
> If you share information between two entities, you may need some 
> ordering so the information are seen consistently by the consumer 
> side. This can be achieved by using barriers.
>
>> Even more, we have
>> a use case
>> when the buffer is not touched by CPU in DomD and is solely owned by 
>> the HW.
>
> Memory ordering issues are subtle. The fact that one of your use-case 
> works does not imply that barriers are not necessary. I am not saying 
> there are a missing barriers, I am only pointed out potential reasons.
>
> Anyway, I don't think your problem is a missing barriers here. It is 
> more likely because of mismatch memory attributes (see above).
>
> Cheers,
>
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