x86/spec-ctrl: Cease using thunk=lfence on AMD

AMD have updated their Spectre v2 guidance, and lfence/jmp is no longer
considered safe.  AMD are recommending using retpoline everywhere.

Update the default heuristics to never select THUNK_LFENCE.

This is part of XSA-398 / CVE-2021-26401.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 8d03080d2a)

docs/misc/xen-command-line.pandoc

@@ -1960,9 +1960,9 @@ to use.
 
 If Xen was compiled with INDIRECT_THUNK support, `bti-thunk=` can be used to
 select which of the thunks gets patched into the `__x86_indirect_thunk_%reg`
-locations.  The default thunk is `retpoline` (generally preferred for Intel
-hardware), with the alternatives being `jmp` (a `jmp *%reg` gadget, minimal
-overhead), and `lfence` (an `lfence; jmp *%reg` gadget, preferred for AMD).
+locations.  The default thunk is `retpoline` (generally preferred), with the
+alternatives being `jmp` (a `jmp *%reg` gadget, minimal overhead), and
+`lfence` (an `lfence; jmp *%reg` gadget).
 
 On hardware supporting IBRS (Indirect Branch Restricted Speculation), the
 `ibrs=` option can be used to force or prevent Xen using the feature itself.

xen/arch/x86/spec_ctrl.c

@@ -904,16 +904,10 @@ void __init init_speculation_mitigations(void)
         if ( IS_ENABLED(CONFIG_INDIRECT_THUNK) )
         {
             /*
-             * AMD's recommended mitigation is to set lfence as being dispatch
-             * serialising, and to use IND_THUNK_LFENCE.
-             */
-            if ( cpu_has_lfence_dispatch )
-                thunk = THUNK_LFENCE;
-            /*
-             * On Intel hardware, we'd like to use retpoline in preference to
+             * On all hardware, we'd like to use retpoline in preference to
              * IBRS, but only if it is safe on this hardware.
              */
-            else if ( retpoline_safe(caps) )
+            if ( retpoline_safe(caps) )
                 thunk = THUNK_RETPOLINE;
             else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
                 ibrs = true;