Re: [Xen-devel] [PATCH v4 3/3] xen/arm: introduce XENFEAT_grant_map_identity

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 07/28/2014 04:21 PM, Stefano Stabellini wrote:
> On Mon, 28 Jul 2014, Julien Grall wrote:
>> Hi Stefano,
>>
>> On 07/28/2014 11:16 AM, Stefano Stabellini wrote:
>>> The flag specifies that the hypervisor maps a grant page to guest
>>> physical address == machine address of the page in addition to the
>>> normal grant mapping address.
>>>
>>> Frontends are allowed to map the same page multiple times using multiple
>>> grant references. On the backend side it can be difficult to find out
>>> the physical address corresponding to a particular machine address,
>>> especially at the completation of a dma operation. To simplify address
>>
>> completion
>>
>> [..]
>>
>>>          if ( err )
>>>          {
>>> @@ -941,9 +951,18 @@ __gnttab_unmap_common(
>>>          int err = 0;
>>>          mapcount(lgt, rd, op->frame, &wrc, &rdc);
>>>          if ( (wrc + rdc) == 0 )
>>> -            err = iommu_unmap_page(ld, op->frame);
>>> -        else if ( wrc == 0 )
>>> -            err = iommu_map_page(ld, op->frame, op->frame, 
>>> IOMMUF_readable);
>>> +        {
>>> +            if ( is_domain_direct_mapped(ld) )
>>> +                err = arch_grant_unmap_page_identity(ld, op->frame);
>>> +            else
>>> +                err = iommu_unmap_page(ld, op->frame);
>>> +        } else if ( wrc == 0 )
>>> +        {
>>> +            if ( is_domain_direct_mapped(ld) )
>>> +                err = arch_grant_map_page_identity(ld, op->frame, 0);
>>> +            else
>>> +                err = iommu_map_page(ld, op->frame, op->frame, 
>>> IOMMUF_readable);
>>
>> With this solution, iommu_map_page will never be called on ARM. It's
>> because gnttab_need_iommu_mapping is checking the domain is using direct
>> mapping.
>>
>> So, I think you can drop the iommu_map_page callback of the SMMU in
>> patch #2.
> 
> OK. Should I keep your acked-by on that patch?

Yes, I'm fine with one of 2 solutions (i.e either keep as it is or
removing the {,un}map_page callback).

Regards,

-- 
Julien Grall

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