[RFC PATCH 2/2] xen/grant-table: Use unpopulated DMAable pages instead of real RAM ones

[Oleksandr Tyshchenko <olekstysh@gmail.com>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

Depends on CONFIG_XEN_UNPOPULATED_ALLOC. If enabled then unpopulated
DMAable (contiguous) pages will be allocated for grant mapping into
instead of ballooning out real RAM pages.

TODO: Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages()
fails.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
---
 drivers/xen/grant-table.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index 8ccccac..2bb4392 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -864,6 +864,25 @@ EXPORT_SYMBOL_GPL(gnttab_free_pages);
  */
 int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
 {
+#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
+	int ret;
+
+	ret = xen_alloc_unpopulated_dma_pages(args->dev, args->nr_pages,
+			args->pages);
+	if (ret < 0)
+		return ret;
+
+	ret = gnttab_pages_set_private(args->nr_pages, args->pages);
+	if (ret < 0) {
+		gnttab_dma_free_pages(args);
+		return ret;
+	}
+
+	args->vaddr = page_to_virt(args->pages[0]);
+	args->dev_bus_addr = page_to_phys(args->pages[0]);
+
+	return ret;
+#else
 	unsigned long pfn, start_pfn;
 	size_t size;
 	int i, ret;
@@ -910,6 +929,7 @@ int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
 fail:
 	gnttab_dma_free_pages(args);
 	return ret;
+#endif
 }
 EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
 
@@ -919,6 +939,12 @@ EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
  */
 int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
 {
+#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
+	gnttab_pages_clear_private(args->nr_pages, args->pages);
+	xen_free_unpopulated_dma_pages(args->dev, args->nr_pages, args->pages);
+
+	return 0;
+#else
 	size_t size;
 	int i, ret;
 
@@ -946,6 +972,7 @@ int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
 		dma_free_wc(args->dev, size,
 			    args->vaddr, args->dev_bus_addr);
 	return ret;
+#endif
 }
 EXPORT_SYMBOL_GPL(gnttab_dma_free_pages);
 #endif
-- 
2.7.4