Re: [Xen-devel] [PATCH] drm/xen-front: Fix mmap attributes for display buffers

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

On 1/29/19 9:07 PM, Julien Grall wrote:
> Hi Oleksandr,
>
> On 1/29/19 3:04 PM, Oleksandr Andrushchenko wrote:
>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
>>
>> When GEM backing storage is allocated those are normal pages,
>> so there is no point using pgprot_writecombine while mmaping.
>> This fixes mismatch of buffer pages' memory attributes between
>> the frontend and backend which may cause screen artifacts.
>>
>> Fixes: c575b7eeb89f ("drm/xen-front: Add support for Xen PV display 
>> frontend")
>>
>> Signed-off-by: Oleksandr Andrushchenko 
>> <oleksandr_andrushchenko@xxxxxxxx>
>> Suggested-by: Julien Grall <julien.grall@xxxxxxx>
>> ---
>>   drivers/gpu/drm/xen/xen_drm_front_gem.c | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/xen/xen_drm_front_gem.c 
>> b/drivers/gpu/drm/xen/xen_drm_front_gem.c
>> index d303a2e17f5e..9d5c03d7668d 100644
>> --- a/drivers/gpu/drm/xen/xen_drm_front_gem.c
>> +++ b/drivers/gpu/drm/xen/xen_drm_front_gem.c
>> @@ -235,8 +235,7 @@ static int gem_mmap_obj(struct xen_gem_object 
>> *xen_obj,
>>       vma->vm_flags &= ~VM_PFNMAP;
>>       vma->vm_flags |= VM_MIXEDMAP;
>>       vma->vm_pgoff = 0;
>> -    vma->vm_page_prot =
>> - pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
>> +    vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
>
> The patch looks good to me. It would be worth expanding the comment a 
> bit before to explain that we overwrite vm_page_prot to use cacheable 
> attribute as required by the Xen ABI.
>
Ok, then I'll put:

+       /*
+        * According to Xen on ARM ABI (xen/include/public/arch-arm.h):
+        * 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.
+        */
         vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
Please let me know if this is not what you want
> With the comment updated:
>
> Acked-by: Julien Grall <julien.grall@xxxxxxx>
>
> Cheers,
>
Thank you,
Oleksandr
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