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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 8 Jul 2014, Ian Campbell wrote:
> 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).

That is true, but in such a system we would have to tell the kernel that
DMAing is safe, so this will turn into:

if (!xen_feature(XENFEAT_grant_map_11) &&
    !xen_feature(XENFEAT_safe_dma)) {
        pr_warn("Please upgrade your Xen.\n"
                        "If your platform has any non-coherent DMA devices, 
they won't work properly.\n");
}

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