xen/arm: introduce XENFEAT_direct_mapped and XENFEAT_not_direct_mapped

Newer Xen versions expose two Xen feature flags to tell us if the domain
is directly mapped or not. Only when a domain is directly mapped it
makes sense to enable swiotlb-xen on ARM.

Introduce a function on ARM to check the new Xen feature flags and also
to deal with the legacy case. Call the function xen_swiotlb_detect.
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20210319200140.12512-1-sstabellini@kernel.orgSigned-off-by: Juergen Gross <jgross@suse.com>

arch/arm/include/asm/xen/swiotlb-xen.h

+#include <xen/arm/swiotlb-xen.h>

arch/arm/xen/mm.c

@@ -135,10 +135,22 @@ void xen_destroy_contiguous_region(phys_addr_t pstart, unsigned int order)
 	return;
 }
 
+int xen_swiotlb_detect(void)
+{
+	if (!xen_domain())
+		return 0;
+	if (xen_feature(XENFEAT_direct_mapped))
+		return 1;
+	/* legacy case */
+	if (!xen_feature(XENFEAT_not_direct_mapped) && xen_initial_domain())
+		return 1;
+	return 0;
+}
+
 static int __init xen_mm_init(void)
 {
 	struct gnttab_cache_flush cflush;
-	if (!xen_initial_domain())
+	if (!xen_swiotlb_detect())
 		return 0;
 	xen_swiotlb_init(1, false);
 

arch/arm64/include/asm/xen/swiotlb-xen.h

+#include <xen/arm/swiotlb-xen.h>

arch/arm64/mm/dma-mapping.c

@@ -53,7 +53,7 @@ void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
 		iommu_setup_dma_ops(dev, dma_base, size);
 
 #ifdef CONFIG_XEN
-	if (xen_initial_domain())
+	if (xen_swiotlb_detect())
 		dev->dma_ops = &xen_swiotlb_dma_ops;
 #endif
 }

include/xen/arm/swiotlb-xen.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_ARM_SWIOTLB_XEN_H
+#define _ASM_ARM_SWIOTLB_XEN_H
+
+extern int xen_swiotlb_detect(void);
+
+#endif /* _ASM_ARM_SWIOTLB_XEN_H */

include/xen/interface/features.h

@@ -83,6 +83,20 @@
  */
 #define XENFEAT_linux_rsdp_unrestricted   15
 
+/*
+ * A direct-mapped (or 1:1 mapped) domain is a domain for which its
+ * local pages have gfn == mfn. If a domain is direct-mapped,
+ * XENFEAT_direct_mapped is set; otherwise XENFEAT_not_direct_mapped
+ * is set.
+ *
+ * If neither flag is set (e.g. older Xen releases) the assumptions are:
+ * - not auto_translated domains (x86 only) are always direct-mapped
+ * - on x86, auto_translated domains are not direct-mapped
+ * - on ARM, Dom0 is direct-mapped, DomUs are not
+ */
+#define XENFEAT_not_direct_mapped         16
+#define XENFEAT_direct_mapped             17
+
 #define XENFEAT_NR_SUBMAPS 1
 
 #endif /* __XEN_PUBLIC_FEATURES_H__ */

include/xen/swiotlb-xen.h

@@ -3,6 +3,7 @@
 #define __LINUX_SWIOTLB_XEN_H
 
 #include <linux/swiotlb.h>
+#include <asm/xen/swiotlb-xen.h>
 
 void xen_dma_sync_for_cpu(struct device *dev, dma_addr_t handle,
 			  size_t size, enum dma_data_direction dir);