SUPPORT.md: Un-shimmed 32-bit PV guests are no longer supported

The support status of 32-bit guests doesn't seem particularly useful.

With it changed to fully unsupported outside of PV-shim, adjust the PV32
Kconfig default accordingly.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
master commit: 1a0f2fe229
master date: 2021-06-04 17:24:05 +0100

SUPPORT.md

@@ -82,14 +82,7 @@ No hardware requirements
 
     Status, x86_64: Supported
     Status, x86_32, shim: Supported
-    Status, x86_32, without shim: Supported, with caveats
-
-Due to architectural limitations,
-32-bit PV guests must be assumed to be able to read arbitrary host memory
-using speculative execution attacks.
-Advisories will continue to be issued
-for new vulnerabilities related to un-shimmed 32-bit PV guests
-enabling denial-of-service attacks or privilege escalation attacks.
+    Status, x86_32, without shim: Supported, not security supported
 
 ### x86/HVM
 

xen/arch/x86/Kconfig

@@ -56,7 +56,7 @@ config PV
 config PV32
 	bool "Support for 32bit PV guests"
 	depends on PV
-	default y
+	default PV_SHIM
 	---help---
 	  The 32bit PV ABI uses Ring1, an area of the x86 architecture which
 	  was deprecated and mostly removed in the AMD64 spec.  As a result,
@@ -67,7 +67,10 @@ config PV32
 	  reduction, or performance reasons.  Backwards compatibility can be
 	  provided via the PV Shim mechanism.
 
-	  If unsure, say Y.
+	  Note that outside of PV Shim, 32-bit PV guests are not security
+	  supported anymore.
+
+	  If unsure, use the default setting.
 
 config PV_LINEAR_PT
        bool "Support for PV linear pagetables"