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>
(cherry picked from commit f2cac32029)

SUPPORT.md

@@ -353,6 +353,16 @@ 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
@@ -485,7 +495,7 @@ 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/build.yaml

@@ -312,11 +312,15 @@ debian-12-x86_64-gcc-debug:
   extends: .gcc-x86-64-build-debug
   variables:
     CONTAINER: debian:12-x86_64
+    EXTRA_XEN_CONFIG: |
+      CONFIG_SHADOW_PAGING=y
 
 debian-12-x86_64-clang-debug:
   extends: .clang-x86-64-build-debug
   variables:
     CONTAINER: debian:12-x86_64
+    EXTRA_XEN_CONFIG: |
+      CONFIG_SHADOW_PAGING=y
 
 debian-12-ppc64le-gcc-debug:
   extends: .gcc-ppc64le-cross-build-debug

xen/arch/x86/Kconfig

@@ -177,7 +177,6 @@ config XEN_IBT
 
 config SHADOW_PAGING
 	bool "Shadow Paging"
-	default !PV_SHIM_EXCLUSIVE
 	depends on PV || HVM
 	help
 
@@ -193,7 +192,8 @@ config SHADOW_PAGING
           Under a small number of specific workloads, shadow paging may be
           deliberately used as a performance optimisation.
 
-          If unsure, say Y.
+	  NOTE: This feature is now deprecated. It is in particular no longer
+	        security supported.
 
 config BIGMEM
 	bool "big memory support"