xen: Make clear that swiotlb and biomerge are dealing with DMA address

The swiotlb is required when programming a DMA address on ARM when a
device is not protected by an IOMMU.

In this case, the DMA address should always be equal to the machine address.
For DOM0 memory, Xen ensure it by have an identity mapping between the
guest address and host address. However, when mapping a foreign grant
reference, the 1:1 model doesn't work.

For ARM guest, most of the callers of pfn_to_mfn expects to get a GFN
(Guest Frame Number), i.e a PFN (Page Frame Number) from the Linux point
of view given that all ARM guest are auto-translated.

Even though the name pfn_to_mfn is misleading, we need to ensure that
those caller get a GFN and not by mistake a MFN. In pratical, I haven't
seen error related to this but we should fix it for the sake of
correctness.

In order to fix the implementation of pfn_to_mfn on ARM in a follow-up
patch, we have to introduce new helpers to return the DMA from a PFN and
the invert.

On x86, the new helpers will be an alias of pfn_to_mfn and mfn_to_pfn.

The helpers will be used in swiotlb and xen_biovec_phys_mergeable.

This is necessary in the latter because we have to ensure that the
biovec code will not try to merge a biovec using foreign page and
another using Linux memory.

Lastly, the helper mfn_to_local_pfn has been renamed to bfn_to_local_pfn
given that the only usage was in swiotlb.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
index a3804fbe1f36..724bffbde54d 100644
--- a/arch/x86/include/asm/xen/page.h
+++ b/arch/x86/include/asm/xen/page.h
@@ -176,6 +176,10 @@ static inline xpaddr_t machine_to_phys(xmaddr_t machine)
 	return XPADDR(PFN_PHYS(mfn_to_pfn(PFN_DOWN(machine.maddr))) | offset);
 }
 
+/* Pseudo-physical <-> Bus conversion */
+#define pfn_to_bfn(pfn)		pfn_to_mfn(pfn)
+#define bfn_to_pfn(bfn)		mfn_to_pfn(bfn)
+
 /*
  * We detect special mappings in one of two ways:
  *  1. If the MFN is an I/O page then Xen will set the m2p entry
@@ -196,7 +200,7 @@ static inline xpaddr_t machine_to_phys(xmaddr_t machine)
  *      require. In all the cases we care about, the FOREIGN_FRAME bit is
  *      masked (e.g., pfn_to_mfn()) so behaviour there is correct.
  */
-static inline unsigned long mfn_to_local_pfn(unsigned long mfn)
+static inline unsigned long bfn_to_local_pfn(unsigned long mfn)
 {
 	unsigned long pfn;
 
@@ -262,7 +266,7 @@ void make_lowmem_page_readwrite(void *vaddr);
 
 static inline bool xen_arch_need_swiotlb(struct device *dev,
 					 unsigned long pfn,
-					 unsigned long mfn)
+					 unsigned long bfn)
 {
 	return false;
 }