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

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

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?
Cheers,

--
Julien Grall