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

[Julien Grall <julien.grall@linaro.org>]

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.

> +                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?

Hence the name is confusing. You may think that we don't need identity
mapping when IOMMU is used while it's actually the case, but we use a
different function.

-- 
Julien Grall