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

[Julien Grall <julien.grall@xxxxxxxxxx>]

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.

Regards,

-- 
Julien Grall

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