Re: [Xen-devel] [PATCH v2 1/3] xen/arm: introduce XENFEAT_grant_map_11

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-07-08 at 16:42 +0100, Stefano Stabellini wrote:
> The flag tells us that the hypervisor maps a grant page to guest
> physical address == machine address of the page in addition to the
> normal grant mapping address. It is needed to properly issue cache
> maintenance operation at the completion of a DMA operation involving a
> foreign grant.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> ---
>  arch/arm/xen/enlighten.c         |    6 ++++++
>  include/xen/interface/features.h |    3 +++
>  2 files changed, 9 insertions(+)
> 
> diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
> index b96723e..ee3135a 100644
> --- a/arch/arm/xen/enlighten.c
> +++ b/arch/arm/xen/enlighten.c
> @@ -262,6 +262,12 @@ static int __init xen_guest_init(void)
>       xen_domain_type = XEN_HVM_DOMAIN;
>  
>       xen_setup_features();
> +
> +     if (!xen_feature(XENFEAT_grant_map_11)) {
> +             pr_warn("Please upgrade your Xen.\n"
> +                             "If your platform has any non-coherent DMA 
> devices, they won't work properly.\n");
> +     }

Unfortunately this isn't quite complete. On a system where all devices
are behind an SMMU then we would want to be able to disable the 1:1
workaround, which in turn would imply disabling this feature flag too
(since it is no longer necessary and also impossible to implement in
that case).

Ian.


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