KVM: SVM: let alternatives handle the cases when RSB filling is required

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 117cc7a908c8
("x86/retpoline: Fill return stack buffer on vmexit").

Later, X86_FEATURE_RSB_VMEXIT was added in commit 9756bba28470
("x86/speculation: Fill RSB on vmexit for IBRS") to handle stuffing the
RSB if RETPOLINE=y *or* KERNEL_IBRS=y, i.e. to also stuff the RSB if the
kernel is configured to do IBRS mitigations on entry/exit.

The AutoIBRS (on AMD) feature implementation added in commit e7862eda309e
("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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[sean: massage changeloge, drop comment about AMD not needing RSB_VMEXIT_LITE]
Signed-off-by: Sean Christopherson <[email protected]>

diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
index a0c8eb37d3e1c62dc4390e6b61ccbbeff6845bff..2ed80aea3bb1307a479884368abe517c33b88b15 100644 (file)
--- a/arch/x86/kvm/svm/vmenter.S
+++ b/arch/x86/kvm/svm/vmenter.S
@@ -209,10 +209,8 @@ 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
+       FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT
 
        /* Clobbers RAX, RCX, RDX.  */
        RESTORE_HOST_SPEC_CTRL
@@ -348,10 +346,8 @@ 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
+       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