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>

2 files changed, 11 insertions, 11 deletions

diff --git a/drivers/xen/biomerge.c b/drivers/xen/biomerge.c
index 0edb91c0de6b..8ae2fc90e1ea 100644
--- a/drivers/xen/biomerge.c
+++ b/drivers/xen/biomerge.c
@@ -6,10 +6,10 @@
 bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
 			       const struct bio_vec *vec2)
 {
-	unsigned long mfn1 = pfn_to_mfn(page_to_pfn(vec1->bv_page));
-	unsigned long mfn2 = pfn_to_mfn(page_to_pfn(vec2->bv_page));
+	unsigned long bfn1 = pfn_to_bfn(page_to_pfn(vec1->bv_page));
+	unsigned long bfn2 = pfn_to_bfn(page_to_pfn(vec2->bv_page));
 
 	return __BIOVEC_PHYS_MERGEABLE(vec1, vec2) &&
-		((mfn1 == mfn2) || ((mfn1+1) == mfn2));
+		((bfn1 == bfn2) || ((bfn1+1) == bfn2));
 }
 EXPORT_SYMBOL(xen_biovec_phys_mergeable);
diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
index 4c549323c605..d757a3e610c6 100644
--- a/drivers/xen/swiotlb-xen.c
+++ b/drivers/xen/swiotlb-xen.c
@@ -82,8 +82,8 @@ static u64 start_dma_addr;
  */
 static inline dma_addr_t xen_phys_to_bus(phys_addr_t paddr)
 {
-	unsigned long mfn = pfn_to_mfn(PFN_DOWN(paddr));
-	dma_addr_t dma = (dma_addr_t)mfn << PAGE_SHIFT;
+	unsigned long bfn = pfn_to_bfn(PFN_DOWN(paddr));
+	dma_addr_t dma = (dma_addr_t)bfn << PAGE_SHIFT;
 
 	dma |= paddr & ~PAGE_MASK;
 
@@ -92,7 +92,7 @@ static inline dma_addr_t xen_phys_to_bus(phys_addr_t paddr)
 
 static inline phys_addr_t xen_bus_to_phys(dma_addr_t baddr)
 {
-	unsigned long pfn = mfn_to_pfn(PFN_DOWN(baddr));
+	unsigned long pfn = bfn_to_pfn(PFN_DOWN(baddr));
 	dma_addr_t dma = (dma_addr_t)pfn << PAGE_SHIFT;
 	phys_addr_t paddr = dma;
 
@@ -110,15 +110,15 @@ static int check_pages_physically_contiguous(unsigned long pfn,
 					     unsigned int offset,
 					     size_t length)
 {
-	unsigned long next_mfn;
+	unsigned long next_bfn;
 	int i;
 	int nr_pages;
 
-	next_mfn = pfn_to_mfn(pfn);
+	next_bfn = pfn_to_bfn(pfn);
 	nr_pages = (offset + length + PAGE_SIZE-1) >> PAGE_SHIFT;
 
 	for (i = 1; i < nr_pages; i++) {
-		if (pfn_to_mfn(++pfn) != ++next_mfn)
+		if (pfn_to_bfn(++pfn) != ++next_bfn)
 			return 0;
 	}
 	return 1;
@@ -138,8 +138,8 @@ static inline int range_straddles_page_boundary(phys_addr_t p, size_t size)
 
 static int is_xen_swiotlb_buffer(dma_addr_t dma_addr)
 {
-	unsigned long mfn = PFN_DOWN(dma_addr);
-	unsigned long pfn = mfn_to_local_pfn(mfn);
+	unsigned long bfn = PFN_DOWN(dma_addr);
+	unsigned long pfn = bfn_to_local_pfn(bfn);
 	phys_addr_t paddr;
 
 	/* If the address is outside our domain, it CAN