[PATCH v2] xen: point xen_start_info to a dummy struct for PV onHVM guests

[Stefano Stabellini]

PV on HVM guests don't have a start_info page mapped by Xen, so
xen_start_info is just NULL for them.
That is problem because other parts of the code expect xen_start_info to
point to something valid, for example xen_initial_domain() is defined as
follow since commit 4c071ee5268f7234c3d084b6093bebccc28cdcba ("arm:
initial Xen support"):


#define xen_initial_domain()    (xen_domain() && \
                 xen_start_info->flags & SIF_INITDOMAIN)


Allocate a dummy start_info struct and point xen_start_info to it, as we
do on ARM.
This is not going to change things for PV guests because
xen_start_info is set by arch/x86/xen/xen-head.S:startup_xen.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>


diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index bf788d3..b73e6ed 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -96,7 +96,14 @@ EXPORT_SYMBOL(machine_to_phys_mapping);
 unsigned long  machine_to_phys_nr;
 EXPORT_SYMBOL(machine_to_phys_nr);
 
-struct start_info *xen_start_info;
+/*
+ * Since 'xen_initial_domain' dereferences xen_start_info we need
+ * a dummy structure filled with zeros (for PVHVM guests which don't
+ * have one). For PV guests we do not have to worry about this as the first
+ * few instructions (startup_xen) set it properly.
+ */
+static struct start_info xen_dummy_start_info;
+struct start_info *xen_start_info = &xen_dummy_start_info;
 EXPORT_SYMBOL_GPL(xen_start_info);
 
 struct shared_info xen_dummy_shared_info;
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