xen/grant-dma-ops: Add option to restrict memory access under Xen

Introduce Xen grant DMA-mapping layer which contains special DMA-mapping
routines for providing grant references as DMA addresses to be used by
frontends (e.g. virtio) in Xen guests.

Add the needed functionality by providing a special set of DMA ops
handling the needed grant operations for the I/O pages.

The subsequent commit will introduce the use case for xen-grant DMA ops
layer to enable using virtio devices in Xen guests in a safe manner.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Link: https://lore.kernel.org/r/1654197833-25362-4-git-send-email-olekstysh@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/Kconfig

@@ -335,4 +335,8 @@ config XEN_UNPOPULATED_ALLOC
 	  having to balloon out RAM regions in order to obtain physical memory
 	  space to create such mappings.
 
+config XEN_GRANT_DMA_OPS
+	bool
+	select DMA_OPS
+
 endmenu

drivers/xen/Makefile

@@ -39,3 +39,4 @@ xen-gntalloc-y				:= gntalloc.o
 xen-privcmd-y				:= privcmd.o privcmd-buf.o
 obj-$(CONFIG_XEN_FRONT_PGDIR_SHBUF)	+= xen-front-pgdir-shbuf.o
 obj-$(CONFIG_XEN_UNPOPULATED_ALLOC)	+= unpopulated-alloc.o
+obj-$(CONFIG_XEN_GRANT_DMA_OPS)		+= grant-dma-ops.o

drivers/xen/grant-dma-ops.c

@@ -0,0 +1,312 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Xen grant DMA-mapping layer - contains special DMA-mapping routines
+ * for providing grant references as DMA addresses to be used by frontends
+ * (e.g. virtio) in Xen guests
+ *
+ * Copyright (c) 2021, Juergen Gross <jgross@suse.com>
+ */
+
+#include <linux/module.h>
+#include <linux/dma-map-ops.h>
+#include <linux/of.h>
+#include <linux/pfn.h>
+#include <linux/xarray.h>
+#include <xen/xen.h>
+#include <xen/xen-ops.h>
+#include <xen/grant_table.h>
+
+struct xen_grant_dma_data {
+	/* The ID of backend domain */
+	domid_t backend_domid;
+	/* Is device behaving sane? */
+	bool broken;
+};
+
+static DEFINE_XARRAY(xen_grant_dma_devices);
+
+#define XEN_GRANT_DMA_ADDR_OFF	(1ULL << 63)
+
+static inline dma_addr_t grant_to_dma(grant_ref_t grant)
+{
+	return XEN_GRANT_DMA_ADDR_OFF | ((dma_addr_t)grant << PAGE_SHIFT);
+}
+
+static inline grant_ref_t dma_to_grant(dma_addr_t dma)
+{
+	return (grant_ref_t)((dma & ~XEN_GRANT_DMA_ADDR_OFF) >> PAGE_SHIFT);
+}
+
+static struct xen_grant_dma_data *find_xen_grant_dma_data(struct device *dev)
+{
+	struct xen_grant_dma_data *data;
+
+	xa_lock(&xen_grant_dma_devices);
+	data = xa_load(&xen_grant_dma_devices, (unsigned long)dev);
+	xa_unlock(&xen_grant_dma_devices);
+
+	return data;
+}
+
+/*
+ * DMA ops for Xen frontends (e.g. virtio).
+ *
+ * Used to act as a kind of software IOMMU for Xen guests by using grants as
+ * DMA addresses.
+ * Such a DMA address is formed by using the grant reference as a frame
+ * number and setting the highest address bit (this bit is for the backend
+ * to be able to distinguish it from e.g. a mmio address).
+ *
+ * Note that for now we hard wire dom0 to be the backend domain. In order
+ * to support any domain as backend we'd need to add a way to communicate
+ * the domid of this backend, e.g. via Xenstore, via the PCI-device's
+ * config space or DT/ACPI.
+ */
+static void *xen_grant_dma_alloc(struct device *dev, size_t size,
+				 dma_addr_t *dma_handle, gfp_t gfp,
+				 unsigned long attrs)
+{
+	struct xen_grant_dma_data *data;
+	unsigned int i, n_pages = PFN_UP(size);
+	unsigned long pfn;
+	grant_ref_t grant;
+	void *ret;
+
+	data = find_xen_grant_dma_data(dev);
+	if (!data)
+		return NULL;
+
+	if (unlikely(data->broken))
+		return NULL;
+
+	ret = alloc_pages_exact(n_pages * PAGE_SIZE, gfp);
+	if (!ret)
+		return NULL;
+
+	pfn = virt_to_pfn(ret);
+
+	if (gnttab_alloc_grant_reference_seq(n_pages, &grant)) {
+		free_pages_exact(ret, n_pages * PAGE_SIZE);
+		return NULL;
+	}
+
+	for (i = 0; i < n_pages; i++) {
+		gnttab_grant_foreign_access_ref(grant + i, data->backend_domid,
+				pfn_to_gfn(pfn + i), 0);
+	}
+
+	*dma_handle = grant_to_dma(grant);
+
+	return ret;
+}
+
+static void xen_grant_dma_free(struct device *dev, size_t size, void *vaddr,
+			       dma_addr_t dma_handle, unsigned long attrs)
+{
+	struct xen_grant_dma_data *data;
+	unsigned int i, n_pages = PFN_UP(size);
+	grant_ref_t grant;
+
+	data = find_xen_grant_dma_data(dev);
+	if (!data)
+		return;
+
+	if (unlikely(data->broken))
+		return;
+
+	grant = dma_to_grant(dma_handle);
+
+	for (i = 0; i < n_pages; i++) {
+		if (unlikely(!gnttab_end_foreign_access_ref(grant + i))) {
+			dev_alert(dev, "Grant still in use by backend domain, disabled for further use\n");
+			data->broken = true;
+			return;
+		}
+	}
+
+	gnttab_free_grant_reference_seq(grant, n_pages);
+
+	free_pages_exact(vaddr, n_pages * PAGE_SIZE);
+}
+
+static struct page *xen_grant_dma_alloc_pages(struct device *dev, size_t size,
+					      dma_addr_t *dma_handle,
+					      enum dma_data_direction dir,
+					      gfp_t gfp)
+{
+	void *vaddr;
+
+	vaddr = xen_grant_dma_alloc(dev, size, dma_handle, gfp, 0);
+	if (!vaddr)
+		return NULL;
+
+	return virt_to_page(vaddr);
+}
+
+static void xen_grant_dma_free_pages(struct device *dev, size_t size,
+				     struct page *vaddr, dma_addr_t dma_handle,
+				     enum dma_data_direction dir)
+{
+	xen_grant_dma_free(dev, size, page_to_virt(vaddr), dma_handle, 0);
+}
+
+static dma_addr_t xen_grant_dma_map_page(struct device *dev, struct page *page,
+					 unsigned long offset, size_t size,
+					 enum dma_data_direction dir,
+					 unsigned long attrs)
+{
+	struct xen_grant_dma_data *data;
+	unsigned int i, n_pages = PFN_UP(size);
+	grant_ref_t grant;
+	dma_addr_t dma_handle;
+
+	if (WARN_ON(dir == DMA_NONE))
+		return DMA_MAPPING_ERROR;
+
+	data = find_xen_grant_dma_data(dev);
+	if (!data)
+		return DMA_MAPPING_ERROR;
+
+	if (unlikely(data->broken))
+		return DMA_MAPPING_ERROR;
+
+	if (gnttab_alloc_grant_reference_seq(n_pages, &grant))
+		return DMA_MAPPING_ERROR;
+
+	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);
+	}
+
+	dma_handle = grant_to_dma(grant) + offset;
+
+	return dma_handle;
+}
+
+static void xen_grant_dma_unmap_page(struct device *dev, dma_addr_t dma_handle,
+				     size_t size, enum dma_data_direction dir,
+				     unsigned long attrs)
+{
+	struct xen_grant_dma_data *data;
+	unsigned int i, n_pages = PFN_UP(size);
+	grant_ref_t grant;
+
+	if (WARN_ON(dir == DMA_NONE))
+		return;
+
+	data = find_xen_grant_dma_data(dev);
+	if (!data)
+		return;
+
+	if (unlikely(data->broken))
+		return;
+
+	grant = dma_to_grant(dma_handle);
+
+	for (i = 0; i < n_pages; i++) {
+		if (unlikely(!gnttab_end_foreign_access_ref(grant + i))) {
+			dev_alert(dev, "Grant still in use by backend domain, disabled for further use\n");
+			data->broken = true;
+			return;
+		}
+	}
+
+	gnttab_free_grant_reference_seq(grant, n_pages);
+}
+
+static void xen_grant_dma_unmap_sg(struct device *dev, struct scatterlist *sg,
+				   int nents, enum dma_data_direction dir,
+				   unsigned long attrs)
+{
+	struct scatterlist *s;
+	unsigned int i;
+
+	if (WARN_ON(dir == DMA_NONE))
+		return;
+
+	for_each_sg(sg, s, nents, i)
+		xen_grant_dma_unmap_page(dev, s->dma_address, sg_dma_len(s), dir,
+				attrs);
+}
+
+static int xen_grant_dma_map_sg(struct device *dev, struct scatterlist *sg,
+				int nents, enum dma_data_direction dir,
+				unsigned long attrs)
+{
+	struct scatterlist *s;
+	unsigned int i;
+
+	if (WARN_ON(dir == DMA_NONE))
+		return -EINVAL;
+
+	for_each_sg(sg, s, nents, i) {
+		s->dma_address = xen_grant_dma_map_page(dev, sg_page(s), s->offset,
+				s->length, dir, attrs);
+		if (s->dma_address == DMA_MAPPING_ERROR)
+			goto out;
+
+		sg_dma_len(s) = s->length;
+	}
+
+	return nents;
+
+out:
+	xen_grant_dma_unmap_sg(dev, sg, i, dir, attrs | DMA_ATTR_SKIP_CPU_SYNC);
+	sg_dma_len(sg) = 0;
+
+	return -EIO;
+}
+
+static int xen_grant_dma_supported(struct device *dev, u64 mask)
+{
+	return mask == DMA_BIT_MASK(64);
+}
+
+static const struct dma_map_ops xen_grant_dma_ops = {
+	.alloc = xen_grant_dma_alloc,
+	.free = xen_grant_dma_free,
+	.alloc_pages = xen_grant_dma_alloc_pages,
+	.free_pages = xen_grant_dma_free_pages,
+	.mmap = dma_common_mmap,
+	.get_sgtable = dma_common_get_sgtable,
+	.map_page = xen_grant_dma_map_page,
+	.unmap_page = xen_grant_dma_unmap_page,
+	.map_sg = xen_grant_dma_map_sg,
+	.unmap_sg = xen_grant_dma_unmap_sg,
+	.dma_supported = xen_grant_dma_supported,
+};
+
+void xen_grant_setup_dma_ops(struct device *dev)
+{
+	struct xen_grant_dma_data *data;
+
+	data = find_xen_grant_dma_data(dev);
+	if (data) {
+		dev_err(dev, "Xen grant DMA data is already created\n");
+		return;
+	}
+
+	data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
+	if (!data)
+		goto err;
+
+	/* XXX The dom0 is hardcoded as the backend domain for now */
+	data->backend_domid = 0;
+
+	if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev, data,
+			GFP_KERNEL))) {
+		dev_err(dev, "Cannot store Xen grant DMA data\n");
+		goto err;
+	}
+
+	dev->dma_ops = &xen_grant_dma_ops;
+
+	return;
+
+err:
+	dev_err(dev, "Cannot set up Xen grant DMA ops, retain platform DMA ops\n");
+}
+
+MODULE_DESCRIPTION("Xen grant DMA-mapping layer");
+MODULE_AUTHOR("Juergen Gross <jgross@suse.com>");
+MODULE_LICENSE("GPL");

include/xen/xen-ops.h

@@ -214,4 +214,12 @@ static inline void xen_preemptible_hcall_end(void) { }
 
 #endif /* CONFIG_XEN_PV && !CONFIG_PREEMPTION */
 
+#ifdef CONFIG_XEN_GRANT_DMA_OPS
+void xen_grant_setup_dma_ops(struct device *dev);
+#else
+static inline void xen_grant_setup_dma_ops(struct device *dev)
+{
+}
+#endif /* CONFIG_XEN_GRANT_DMA_OPS */
+
 #endif /* INCLUDE_XEN_OPS_H */