[PATCH v2] xen: introduce XENFEAT_direct_mapped and XENFEAT_not_direct_mapped

[Stefano Stabellini <sstabellini@kernel.org>]

Introduce two feature flags to tell the domain whether it is
direct-mapped or not. It allows the guest kernel to make informed
decisions on things such as swiotlb-xen enablement.

The introduction of both flags (XENFEAT_direct_mapped and
XENFEAT_not_direct_mapped) allows the guest kernel to avoid any
guesswork if one of the two is present, or fallback to the current
checks if neither of them is present.

XENFEAT_direct_mapped is always set for not auto-translated guests.

For auto-translated guests, only Dom0 on ARM is direct-mapped. Also,
see is_domain_direct_mapped() which refers to auto-translated guests:
xen/include/asm-arm/domain.h:is_domain_direct_mapped
xen/include/asm-x86/domain.h:is_domain_direct_mapped

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
CC: jbeulich@suse.com
CC: andrew.cooper3@citrix.com
CC: julien@xen.org
---
Changes in v2:
- code style improvements
- better comments
- better commit message
- not auto_translated domains are direct_mapped
---
 xen/common/kernel.c           |  4 ++++
 xen/include/public/features.h | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index 7a345ae45e..431447326c 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -560,6 +560,10 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
                              (1U << XENFEAT_hvm_callback_vector) |
                              (has_pirq(d) ? (1U << XENFEAT_hvm_pirqs) : 0);
 #endif
+            if ( is_domain_direct_mapped(d) || !paging_mode_translate(d) )
+                fi.submap |= (1U << XENFEAT_direct_mapped);
+            else
+                fi.submap |= (1U << XENFEAT_not_direct_mapped);
             break;
         default:
             return -EINVAL;
diff --git a/xen/include/public/features.h b/xen/include/public/features.h
index 1613b2aab8..4aebfd359a 100644
--- a/xen/include/public/features.h
+++ b/xen/include/public/features.h
@@ -114,6 +114,18 @@
  */
 #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.
+ *
+ * Not auto_translated domains are always direct-mapped. Also see
+ * XENFEAT_auto_translated_physmap.
+ */
+#define XENFEAT_not_direct_mapped         16
+#define XENFEAT_direct_mapped             17
+
 #define XENFEAT_NR_SUBMAPS 1
 
 #endif /* __XEN_PUBLIC_FEATURES_H__ */
-- 
2.17.1