From: Amit Shah <amit.shah@xxxxxxx>
Remove superfluous RSB filling after a VMEXIT when the CPU already has
flushed the RSB after a VMEXIT when AutoIBRS is enabled.
The initial implementation for adding RETPOLINES added an ALTERNATIVES
implementation for filling the RSB after a VMEXIT in
commit 117cc7a908c836 ("x86/retpoline: Fill return stack buffer on vmexit")
Later, X86_FEATURE_RSB_VMEXIT was added in
commit 2b129932201673 ("x86/speculation: Add RSB VM Exit protections")
The AutoIBRS (on AMD CPUs) feature implementation added in
commit e7862eda309ecf ("x86/cpu: Support AMD Automatic IBRS")
used the already-implemented logic for EIBRS in
spectre_v2_determine_rsb_fill_type_on_vmexit() -- but did not update the
code at VMEXIT to act on the mode selected in that function -- resulting
in VMEXITs continuing to clear the RSB when RETPOLINES are enabled,
despite the presence of AutoIBRS.
Signed-off-by: Amit Shah <amit.shah@xxxxxxx>
---
v4: resend of v3 with subject-prefix fixed
v3:
- Add a comment mentioning SVM does not need RSB_VMEXIT_LITE unlike
VMX.
v2:
- tweak commit message re: Boris's comments.
arch/x86/kvm/svm/vmenter.S | 24 ++++++++++++++++--------
1 file changed, 16 insertions(+), 8 deletions(-)
diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
index a0c8eb37d3e1..69d9825ebdd9 100644
--- a/arch/x86/kvm/svm/vmenter.S
+++ b/arch/x86/kvm/svm/vmenter.S
@@ -209,10 +209,14 @@ SYM_FUNC_START(__svm_vcpu_run)
7: vmload %_ASM_AX
8:
-#ifdef CONFIG_MITIGATION_RETPOLINE
- /* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
- FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
-#endif
+ /*
+ * IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET!
+ *
+ * Unlike VMX, AMD does not have the hardware bug that necessitates
+ * RSB_VMEXIT_LITE
+ */
+
+ FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT
/* Clobbers RAX, RCX, RDX. */
RESTORE_HOST_SPEC_CTRL
@@ -348,10 +352,14 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)
2: cli
-#ifdef CONFIG_MITIGATION_RETPOLINE
- /* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
- FILL_RETURN_BUFFER %rax, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
-#endif
+ /*
+ * IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET!
+ *
+ * Unlike VMX, AMD does not have the hardware bug that necessitates
+ * RSB_VMEXIT_LITE
+ */
+
+ FILL_RETURN_BUFFER %rax, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT
/* Clobbers RAX, RCX, RDX, consumes RDI (@svm) and RSI (@spec_ctrl_intercepted). */
RESTORE_HOST_SPEC_CTRL
--
2.45.2
