[xen master] x86/spec-ctrl: Build with BRANCH_HARDEN lfences by default

[patchbot@xxxxxxx]

commit b11380f6cd58ce40422473262b2b4c2b210447a1
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Oct 4 21:39:03 2021 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Oct 8 18:52:22 2021 +0100

    x86/spec-ctrl: Build with BRANCH_HARDEN lfences by default
    
    Branch Harden is enabled by default at compile and boot time.  Invert the
    logic to compile with lfence by default and nop out in the non-default case.
    
    This has several advantages.  It removes 3829 patch points (in the random
    build of Xen I have to hand) by default on boot, 70% (!) of the
    .altinstr_replacement section.  For builds of Xen with a non-nops capable 
tool
    chain, the code after `spec-ctrl=no-branch-harden` is better because Xen can
    write long nops.
    
    Most importantly however, it means the disassembly actually matches what 
runs
    in the common case, with the ability to distinguish the lfences from other
    uses of nops.
    
    Finally, make opt_branch_harden local to spec_ctrl.c and __initdata.  It has
    never been used externally, even at it's introduction in c/s 3860d5534df4
    "spec: add l1tf-barrier".
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/spec_ctrl.c          | 7 ++++---
 xen/include/asm-x86/cpufeatures.h | 2 +-
 xen/include/asm-x86/nospec.h      | 2 +-
 xen/include/asm-x86/spec_ctrl.h   | 1 -
 4 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 16d2a1d172..a5569c7f2b 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -52,7 +52,7 @@ bool __read_mostly opt_ibpb = true;
 bool __read_mostly opt_ssbd = false;
 int8_t __read_mostly opt_eager_fpu = -1;
 int8_t __read_mostly opt_l1d_flush = -1;
-bool __read_mostly opt_branch_harden = true;
+static bool __initdata opt_branch_harden = true;
 
 bool __initdata bsp_delay_spec_ctrl;
 uint8_t __read_mostly default_xen_spec_ctrl;
@@ -1121,8 +1121,9 @@ void __init init_speculation_mitigations(void)
     else if ( opt_l1d_flush == -1 )
         opt_l1d_flush = cpu_has_bug_l1tf && !(caps & ARCH_CAPS_SKIP_L1DFL);
 
-    if ( opt_branch_harden )
-        setup_force_cpu_cap(X86_FEATURE_SC_BRANCH_HARDEN);
+    /* We compile lfence's in by default, and nop them out if requested. */
+    if ( !opt_branch_harden )
+        setup_force_cpu_cap(X86_FEATURE_SC_NO_BRANCH_HARDEN);
 
     /*
      * We do not disable HT by default on affected hardware.
diff --git a/xen/include/asm-x86/cpufeatures.h 
b/xen/include/asm-x86/cpufeatures.h
index 72beb7babc..b10154fc44 100644
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -27,7 +27,7 @@ XEN_CPUFEATURE(XEN_SMAP,          X86_SYNTH(11)) /* SMAP gets 
used by Xen itself
 /* Bit 12 - unused. */
 XEN_CPUFEATURE(IND_THUNK_LFENCE,  X86_SYNTH(13)) /* Use IND_THUNK_LFENCE */
 XEN_CPUFEATURE(IND_THUNK_JMP,     X86_SYNTH(14)) /* Use IND_THUNK_JMP */
-XEN_CPUFEATURE(SC_BRANCH_HARDEN,  X86_SYNTH(15)) /* Conditional Branch 
Hardening */
+XEN_CPUFEATURE(SC_NO_BRANCH_HARDEN, X86_SYNTH(15)) /* (Disable) Conditional 
branch hardening */
 XEN_CPUFEATURE(SC_MSR_PV,         X86_SYNTH(16)) /* MSR_SPEC_CTRL used by Xen 
for PV */
 XEN_CPUFEATURE(SC_MSR_HVM,        X86_SYNTH(17)) /* MSR_SPEC_CTRL used by Xen 
for HVM */
 XEN_CPUFEATURE(SC_RSB_PV,         X86_SYNTH(18)) /* RSB overwrite needed for 
PV */
diff --git a/xen/include/asm-x86/nospec.h b/xen/include/asm-x86/nospec.h
index f6eb84eee5..5312ae4c6f 100644
--- a/xen/include/asm-x86/nospec.h
+++ b/xen/include/asm-x86/nospec.h
@@ -10,7 +10,7 @@
 static always_inline bool barrier_nospec_true(void)
 {
 #ifdef CONFIG_SPECULATIVE_HARDEN_BRANCH
-    alternative("", "lfence", X86_FEATURE_SC_BRANCH_HARDEN);
+    alternative("lfence", "", X86_FEATURE_SC_NO_BRANCH_HARDEN);
 #endif
     return true;
 }
diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
index e671b6fd8d..a803d16f90 100644
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -37,7 +37,6 @@ extern bool opt_ibpb;
 extern bool opt_ssbd;
 extern int8_t opt_eager_fpu;
 extern int8_t opt_l1d_flush;
-extern bool opt_branch_harden;
 
 extern bool bsp_delay_spec_ctrl;
 extern uint8_t default_xen_spec_ctrl;
--
generated by git-patchbot for /home/xen/git/xen.git#master