Re: [PATCH 2/3] gnttab: bypass IOMMU (un)mapping when a domain is (un)mapping its own grant

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.02.2021 12:03, Julien Grall wrote:
> On 17/02/2021 10:46, Jan Beulich wrote:
>> Mappings for a domain's own pages should already be present in the
>> IOMMU. While installing the same mapping again is merely redundant (and
>> inefficient), removing the mapping when the grant mapping gets removed
>> is outright wrong in this case: The mapping was there before the map, so
>> should remain in place after unmapping.
>>
>> This affects
>> - Arm Dom0 in the direct mapped case,
>> - x86 PV Dom0 in the "iommu=dom0-strict" / "dom0-iommu=strict" cases,
>> - all x86 PV DomU-s, including driver domains.
>>
>> Reported-by: Rahul Singh <Rahul.Singh@xxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>> --- a/xen/common/grant_table.c
>> +++ b/xen/common/grant_table.c
>> @@ -1243,7 +1243,7 @@ map_grant_ref(
>>           goto undo_out;
>>       }
>>   
>> -    need_iommu = gnttab_need_iommu_mapping(ld);
>> +    need_iommu = ld != rd && gnttab_need_iommu_mapping(ld);
> 
> AFAICT, the owner of the page may not always be rd. So do we want to 
> check against the owner instead?

For the DomIO case - specifically not. And the DomCOW case can't
happen when an IOMMU is in use. Did I overlook any other cases
where the page may not be owned by rd?

Jan