xen/virtio: Enable restricted memory access using Xen grant mappings

In order to support virtio in Xen guests add a config option XEN_VIRTIO
enabling the user to specify whether in all Xen guests virtio should
be able to access memory via Xen grant mappings only on the host side.

Also set PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS feature from the guest
initialization code on Arm and x86 if CONFIG_XEN_VIRTIO is enabled.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/1654197833-25362-5-git-send-email-olekstysh@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>

arch/arm/xen/enlighten.c

@@ -443,6 +443,8 @@ static int __init xen_guest_init(void)
 	if (!xen_domain())
 		return 0;
 
+	xen_set_restricted_virtio_memory_access();
+
 	if (!acpi_disabled)
 		xen_acpi_guest_init();
 	else

arch/x86/xen/enlighten_hvm.c

@@ -195,6 +195,8 @@ static void __init xen_hvm_guest_init(void)
 	if (xen_pv_domain())
 		return;
 
+	xen_set_restricted_virtio_memory_access();
+
 	init_hvm_pv_info();
 
 	reserve_shared_info();

arch/x86/xen/enlighten_pv.c

@@ -109,6 +109,8 @@ static DEFINE_PER_CPU(struct tls_descs, shadow_tls_desc);
 
 static void __init xen_pv_init_platform(void)
 {
+	xen_set_restricted_virtio_memory_access();
+
 	populate_extra_pte(fix_to_virt(FIX_PARAVIRT_BOOTMAP));
 
 	set_fixmap(FIX_PARAVIRT_BOOTMAP, xen_start_info->shared_info);

drivers/xen/Kconfig

@@ -339,4 +339,15 @@ config XEN_GRANT_DMA_OPS
 	bool
 	select DMA_OPS
 
+config XEN_VIRTIO
+	bool "Xen virtio support"
+	depends on VIRTIO
+	select XEN_GRANT_DMA_OPS
+	help
+	  Enable virtio support for running as Xen guest. Depending on the
+	  guest type this will require special support on the backend side
+	  (qemu or kernel, depending on the virtio device types used).
+
+	  If in doubt, say n.
+
 endmenu

include/xen/xen.h

@@ -52,6 +52,14 @@ bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
 extern u64 xen_saved_max_mem_size;
 #endif
 
+#include <linux/platform-feature.h>
+
+static inline void xen_set_restricted_virtio_memory_access(void)
+{
+	if (IS_ENABLED(CONFIG_XEN_VIRTIO) && xen_domain())
+		platform_set(PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS);
+}
+
 #ifdef CONFIG_XEN_UNPOPULATED_ALLOC
 int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages);
 void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages);