x86: SHADOW_PAGING is deprecated

Addressing certain issues, in particular related to operations which may
take excessively long and therefore would need preemption, has turned out
overly costly. Since alternatives (HVM/PVH: HAP, PV: shim) are commonly
available, the decision was to deprecate the functionality, while still
retaining it for people to use at their own (security) risk. Memory-wise
small enough guests may still be okay to run.

Some CI testing depends on SHADOW_PAGING.  Explicitly enable it when needed.

This is CVE-2026-42493 / XSA-495.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

SUPPORT.md

@@ -366,6 +366,16 @@ This is typically done by a guest kernel agent known as a "balloon driver".
 
     Status: Supported
 
+### Shadow paging
+
+Allows fully virtualized guests (HVM / PVH) to be run without (host side) page
+translation support by hardware (AMD: NPT, Intel: EPT).
+
+It is also required to migrate PV guests, and to allow L1TF-vulnerable guests
+to continue to run without compromising host security.
+
+    Status: Supported, not security supported
+
 ### Populate-on-demand memory
 
 This is a mechanism that allows normal operating systems with only a balloon driver
@@ -498,7 +508,7 @@ This feature is independent
 of the ARM "page granularity" feature (see below).
 
     Status, x86 HVM/PVH, HAP: Supported
-    Status, x86 HVM/PVH, Shadow, 2MiB: Supported
+    Status, x86 HVM/PVH, Shadow, 2MiB: Supported, not security supported
     Status, ARM: Supported
 
 On x86 in shadow mode, only 2MiB (L2) superpages are available;

automation/gitlab-ci/analyze.yaml

@@ -59,6 +59,7 @@ eclair-x86_64-allcode:
       CONFIG_PERF_ARRAYS=y
       CONFIG_PERF_COUNTERS=y
       CONFIG_PV32=y
+      CONFIG_SHADOW_PAGING=y
       CONFIG_UNSUPPORTED=y
       CONFIG_XEN_GUEST=y
       CONFIG_XHCI=y
@@ -91,7 +92,6 @@ eclair-x86_64-amd:
       CONFIG_PV=n
       CONFIG_XEN_IBT=n
       CONFIG_XEN_SHSTK=n
-      CONFIG_SHADOW_PAGING=n
       CONFIG_HVM_FEP=n
       CONFIG_TBOOT=n
       CONFIG_HYPFS=n

automation/gitlab-ci/build.yaml

@@ -296,12 +296,16 @@ debian-13-x86_64-gcc-debug:
   <<: *build-test
   variables:
     CONTAINER: debian:13-x86_64
+    EXTRA_XEN_CONFIG: |
+      CONFIG_SHADOW_PAGING=y
 
 debian-13-x86_64-clang-debug:
   extends: .clang-x86_64-build-debug
   <<: *build-test
   variables:
     CONTAINER: debian:13-x86_64
+    EXTRA_XEN_CONFIG: |
+      CONFIG_SHADOW_PAGING=y
 
 debian-13-ppc64le-gcc-debug:
   extends: .gcc-ppc64le-cross-build-debug

xen/arch/x86/Kconfig

@@ -145,7 +145,6 @@ config XEN_IBT
 
 config SHADOW_PAGING
 	bool "Shadow Paging"
-	default !PV_SHIM_EXCLUSIVE
 	depends on PV || HVM
 	help
 	  Shadow paging is a software alternative to hardware paging support
@@ -160,6 +159,9 @@ config SHADOW_PAGING
 	  Under a small number of specific workloads, shadow paging may be
 	  deliberately used as a performance optimisation.
 
+	  NOTE: This feature is now deprecated. It is in particular no longer
+	        security supported.
+
 config PAGING
 	def_bool HVM || SHADOW_PAGING