xen/swiotlb: Add support for 64KB page granularity

Swiotlb is used on ARM64 to support DMA on platform where devices are
not protected by an SMMU. Furthermore it's only enabled for DOM0.

While Xen is always using 4KB page granularity in the stage-2 page table,
Linux ARM64 may either use 4KB or 64KB. This means that a Linux page
can be spanned accross multiple Xen page.

The Swiotlb code has to validate that the buffer used for DMA is
physically contiguous in the memory. As a Linux page can't be shared
between local memory and foreign page by design (the balloon code always
removing entirely a Linux page), the changes in the code are very
minimal because we only need to check the first Xen PFN.

Note that it may be possible to optimize the function
check_page_physically_contiguous to avoid looping over every Xen PFN
for local memory. Although I will let this optimization for a follow-up.

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, 17 insertions, 9 deletions

diff --git a/arch/arm/include/asm/xen/page-coherent.h b/arch/arm/include/asm/xen/page-coherent.h
index efd562412850..0375c8caa061 100644
--- a/arch/arm/include/asm/xen/page-coherent.h
+++ b/arch/arm/include/asm/xen/page-coherent.h
@@ -35,11 +35,15 @@ static inline void xen_dma_map_page(struct device *hwdev, struct page *page,
 	     dma_addr_t dev_addr, unsigned long offset, size_t size,
 	     enum dma_data_direction dir, struct dma_attrs *attrs)
 {
-	bool local = PFN_DOWN(dev_addr) == page_to_pfn(page);
-	/* Dom0 is mapped 1:1, so if pfn == mfn the page is local otherwise
-	 * is a foreign page grant-mapped in dom0. If the page is local we
-	 * can safely call the native dma_ops function, otherwise we call
-	 * the xen specific function. */
+	bool local = XEN_PFN_DOWN(dev_addr) == page_to_xen_pfn(page);
+	/*
+	 * Dom0 is mapped 1:1, while the Linux page can be spanned accross
+	 * multiple Xen page, it's not possible to have a mix of local and
+	 * foreign Xen page. So if the first xen_pfn == mfn the page is local
+	 * otherwise it's a foreign page grant-mapped in dom0. If the page is
+	 * local we can safely call the native dma_ops function, otherwise we
+	 * call the xen specific function.
+	 */
 	if (local)
 		__generic_dma_ops(hwdev)->map_page(hwdev, page, offset, size, dir, attrs);
 	else
@@ -51,10 +55,14 @@ static inline void xen_dma_unmap_page(struct device *hwdev, dma_addr_t handle,
 		struct dma_attrs *attrs)
 {
 	unsigned long pfn = PFN_DOWN(handle);
-	/* Dom0 is mapped 1:1, so calling pfn_valid on a foreign mfn will
-	 * always return false. If the page is local we can safely call the
-	 * native dma_ops function, otherwise we call the xen specific
-	 * function. */
+	/*
+	 * Dom0 is mapped 1:1, while the Linux page can be spanned accross
+	 * multiple Xen page, it's not possible to have a mix of local and
+	 * foreign Xen page. Dom0 is mapped 1:1, so calling pfn_valid on a
+	 * foreign mfn will always return false. If the page is local we can
+	 * safely call the native dma_ops function, otherwise we call the xen
+	 * specific function.
+	 */
 	if (pfn_valid(pfn)) {
 		if (__generic_dma_ops(hwdev)->unmap_page)
 			__generic_dma_ops(hwdev)->unmap_page(hwdev, handle, size, dir, attrs);