xen/arm: p2m: Add more debug in get_page_from_gva

The function get_page_from_gva is used by copy_*_guest helpers to
translate a guest virtual address to a machine physical address and take
reference on the page.

There are a couple of errors paths that will return the same value making
it difficult to know the exact error. Add more debug in each error patch
only for debug-build.

This should help narrowing down the intermittent failure with the
hypercall GNTTABOP_copy (see [1]).

[1] https://lists.xen.org/archives/html/xen-devel/2017-11/msg00942.html

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/p2m.c

@@ -1428,16 +1428,29 @@ struct page_info *get_page_from_gva(struct vcpu *v, vaddr_t va,
     par = gvirt_to_maddr(va, &maddr, flags);
 
     if ( par )
+    {
+        dprintk(XENLOG_G_DEBUG,
+                "%pv: gvirt_to_maddr failed va=%#"PRIvaddr" flags=0x%lx par=%#"PRIx64"\n",
+                v, va, flags, par);
         goto err;
+    }
 
     if ( !mfn_valid(maddr_to_mfn(maddr)) )
+    {
+        dprintk(XENLOG_G_DEBUG, "%pv: Invalid MFN %#"PRI_mfn"\n",
+                v, mfn_x(maddr_to_mfn(maddr)));
         goto err;
+    }
 
     page = mfn_to_page(maddr_to_mfn(maddr));
     ASSERT(page);
 
     if ( unlikely(!get_page(page, d)) )
+    {
+        dprintk(XENLOG_G_DEBUG, "%pv: Failing to acquire the MFN %#"PRI_mfn"\n",
+                v, mfn_x(maddr_to_mfn(maddr)));
         page = NULL;
+    }
 
 err:
     if ( !page && p2m->mem_access_enabled )