Re: [Xen-devel] [PATCH v3 3/3] xen/arm: introduce XENFEAT_grant_map_identity

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 07/24/2014 03:14 PM, Stefano Stabellini wrote:
> On Thu, 24 Jul 2014, Julien Grall wrote:
>> Hi Stefano,
>>
>> On 07/24/2014 02:31 PM, Stefano Stabellini wrote:
>>> diff --git a/xen/common/kernel.c b/xen/common/kernel.c
>>> index 7e83353..dacbe38 100644
>>> --- a/xen/common/kernel.c
>>> +++ b/xen/common/kernel.c
>>> @@ -15,6 +15,7 @@
>>>  #include <xen/guest_access.h>
>>>  #include <xen/hypercall.h>
>>>  #include <asm/current.h>
>>> +#include <asm/grant_table.h>
>>>  #include <public/nmi.h>
>>>  #include <public/version.h>
>>>  
>>> @@ -325,6 +326,8 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) 
>>> arg)
>>>                  break;
>>>              }
>>>  #endif
>>> +            if ( gnttab_need_identity_mapping(d) )
>>
>> Actually even platform the IOMMU support needs to have this flags on.
>> With this solution you break platform where not every DMA-capable device
>> are behind an SMMU.
> 
> I see.
> 
> 
>>> +                fi.submap |= 1U << XENFEAT_grant_map_identity;
>>>              break;
>>>          default:
>>>              return -EINVAL;
>>> diff --git a/xen/include/asm-arm/grant_table.h 
>>> b/xen/include/asm-arm/grant_table.h
>>> index eac8a70..6f7ccd9 100644
>>> --- a/xen/include/asm-arm/grant_table.h
>>> +++ b/xen/include/asm-arm/grant_table.h
>>> @@ -36,6 +36,9 @@ static inline int replace_grant_supported(void)
>>>  #define gnttab_need_iommu_mapping(d)                    \
>>>      (is_domain_direct_mapped(d) && need_iommu(d))
>>>  
>>> +#define gnttab_need_identity_mapping(d)                    \
>>> +    (is_domain_direct_mapped(d) && !need_iommu(d))
>>> +
>>
>> Why didn't you drop the need_iommu(d) in is_domain_direct_mapped?
> 
> I guess you mean why didn't you drop the need_iommu(d) in
> gnttab_need_iommu_mapping?
> 
> Because how can you need an iommu mapping if an iommu is not present?

I would invert the name of the 2 macros. And drop !need_iommu(d) in the
second.

The code would look like:

if (gnttab_need_identity_mapping(d))
{
   if (gnttab_need_iommu_mapping(d))
     iommu_map_page
   else
     arch_
}

On x86, the gnttab_need_iommu_mapping would be equal to 1.

Regards,

-- 
Julien Grall

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