xen/virtio: Handle cases when page offset > PAGE_SIZE properly

Passed to xen_grant_dma_map_page() offset in the page
can be > PAGE_SIZE even if the guest uses the same page granularity
as Xen (4KB).

Before current patch, if such case happened we ended up providing
grants for the whole region in xen_grant_dma_map_page() which
was really unnecessary. The more, we ended up not releasing all
grants which represented that region in xen_grant_dma_unmap_page().

Current patch updates the code to be able to deal with such cases.
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Link: https://lore.kernel.org/r/20221008151013.2537826-2-olekstysh@gmail.comSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/grant-dma-ops.c

@@ -168,7 +168,9 @@ static dma_addr_t xen_grant_dma_map_page(struct device *dev, struct page *page,
 					 unsigned long attrs)
 {
 	struct xen_grant_dma_data *data;
-	unsigned int i, n_pages = PFN_UP(offset + size);
+	unsigned long dma_offset = offset_in_page(offset),
+			pfn_offset = PFN_DOWN(offset);
+	unsigned int i, n_pages = PFN_UP(dma_offset + size);
 	grant_ref_t grant;
 	dma_addr_t dma_handle;
 
@@ -187,10 +189,11 @@ static dma_addr_t xen_grant_dma_map_page(struct device *dev, struct page *page,
 
 	for (i = 0; i < n_pages; i++) {
 		gnttab_grant_foreign_access_ref(grant + i, data->backend_domid,
-				xen_page_to_gfn(page) + i, dir == DMA_TO_DEVICE);
+				pfn_to_gfn(page_to_xen_pfn(page) + i + pfn_offset),
+				dir == DMA_TO_DEVICE);
 	}
 
-	dma_handle = grant_to_dma(grant) + offset;
+	dma_handle = grant_to_dma(grant) + dma_offset;
 
 	return dma_handle;
 }