[xen staging-4.12] x86/spec-ctrl: Cease using thunk=lfence on AMD

[patchbot@xxxxxxx]

commit 944afa38d9339a67f0164d07fb7ac8a54e9a4c60
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Mar 7 16:35:52 2022 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Mar 8 17:12:50 2022 +0000

    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@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit 8d03080d2a339840d3a59e0932a94f804e45110d)
---
 docs/misc/xen-command-line.pandoc |  6 +++---
 xen/arch/x86/spec_ctrl.c          | 10 ++--------
 2 files changed, 5 insertions(+), 11 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index d7919f2ed4..f9fb162138 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/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.
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index e2fcefc86a..866b864918 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/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;
--
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