arm/xen: implement correctly pfn_to_mfn

After the commit introducing convertion between DMA and guest addresses,
all the callers of pfn_to_mfn are expecting to get a GFN (Guest Frame
Number). On ARM, all the guests are auto-translated so the GFN is equal
to the Linux PFN (Pseudo-physical Frame Number).

The current implementation may return an MFN if the caller is passing a
PFN associated to a mapped foreign grant. In pratice, I haven't seen
the problem on running guest but we should fix it for the sake of
correctness.

Correct the implementation by always returning the pfn passed in parameter.

A follow-up patch will take care to rename pfn_to_mfn to a suitable
name.

Signed-off-by: Julien Grall <[e-mail]>
Reviewed-by: Stefano Stabellini <[e-mail]>
Signed-off-by: David Vrabel <[e-mail]>

arch/arm/include/asm/xen/page.h

···
 
 static inline unsigned long pfn_to_mfn(unsigned long pfn)
 {
-	unsigned long mfn;
-
-	if (phys_to_mach.rb_node != NULL) {
-		mfn = __pfn_to_mfn(pfn);
-		if (mfn != INVALID_P2M_ENTRY)
-			return mfn;
-	}
-
 	return pfn;
 }