[PATCH v2 7/7] KVM: VMX: replace vmx_spec_ctrl_restore_host with RESTORE_HOST_SPEC_CTRL_BODY

Paolo Bonzini
2026-04-27T10:59:04Z

Reuse the same assembly as SVM, just with alternatives instead
of cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/kvm/vmenter.h | 9 +++++++--
arch/x86/kvm/vmx/vmenter.S | 20 +++++++++++++-------
arch/x86/kvm/vmx/vmx.c | 25 -------------------------
arch/x86/kvm/vmx/vmx.h | 1 -
4 files changed, 20 insertions(+), 35 deletions(-)

diff --git a/arch/x86/kvm/vmenter.h b/arch/x86/kvm/vmenter.h
index 84951e186859..93db912eef44 100644
--- a/arch/x86/kvm/vmenter.h
+++ b/arch/x86/kvm/vmenter.h
@@ -55,7 +55,12 @@
#ifdef CONFIG_X86_64
mov PER_CPU_VAR(x86_spec_ctrl_current), %rdx
cmp \guest_spec_ctrl, %rdx
- je \label
+ /*
+ * For legacy IBRS, the IBRS bit always needs to be written after
+ * transitioning from a less privileged predictor mode, regardless of
+ * whether the guest/host values differ.
+ */
+ ALTERNATIVE __stringify(je \label), "", X86_FEATURE_KERNEL_IBRS
movl %edx, %eax
shr $32, %rdx
#else
@@ -66,7 +71,7 @@
mov 4 + \guest_spec_ctrl, %edi
xor %edx, %edi
or %edi, %esi
- je \label
+ ALTERNATIVE __stringify(je \label), "", X86_FEATURE_KERNEL_IBRS
#endif
wrmsr
.endm
diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 7dd9bac0e454..294407dfc24f 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -105,9 +105,9 @@ SYM_FUNC_START(__vmx_vcpu_run)
* Unlike AMD there's no V_SPEC_CTRL here, so do not leave the body
* out of line. Clobbers RAX, RCX, RDX, RSI.
*/
- ALTERNATIVE "jmp .Lspec_ctrl_done", "", X86_FEATURE_MSR_SPEC_CTRL
- RESTORE_GUEST_SPEC_CTRL_BODY VMX_spec_ctrl(%_ASM_DI), .Lspec_ctrl_done
-.Lspec_ctrl_done:
+ ALTERNATIVE "jmp .Lspec_ctrl_guest_done", "", X86_FEATURE_MSR_SPEC_CTRL
+ RESTORE_GUEST_SPEC_CTRL_BODY VMX_spec_ctrl(%_ASM_DI), .Lspec_ctrl_guest_done
+.Lspec_ctrl_guest_done:

/*
* Since vmentry is serializing on affected CPUs, there's no need for
@@ -252,16 +252,22 @@ SYM_INNER_LABEL_ALIGN(vmx_vmexit, SYM_L_GLOBAL)
FILL_RETURN_BUFFER %_ASM_CX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT,\
X86_FEATURE_RSB_VMEXIT_LITE

- pop %_ASM_ARG2 /* @flags */
- pop %_ASM_ARG1 /* @vmx */
-
- call vmx_spec_ctrl_restore_host
+ /* Clobbers RAX, RCX, RDX, RSI. */
+ ALTERNATIVE "jmp .Lspec_ctrl_host_done", "", X86_FEATURE_MSR_SPEC_CTRL
+ mov WORD_SIZE(%_ASM_SP), %_ASM_DI
+ RESTORE_HOST_SPEC_CTRL_BODY VMX_spec_ctrl(%_ASM_DI), (%_ASM_SP), .Lspec_ctrl_host_done
+.Lspec_ctrl_host_done:

CLEAR_BRANCH_HISTORY_VMEXIT

/* Put return value in AX */
mov %_ASM_BX, %_ASM_AX

+ /* Pop our saved arguments from the stack */
+ pop %_ASM_BX
+ pop %_ASM_BX
+
+ /* ... and then the callee-save registers */
pop %_ASM_BX
#ifdef CONFIG_X86_64
pop %r12
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index a8039b0f9392..b033f611fa04 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7388,31 +7388,6 @@ void noinstr vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp)
}
}

-void noinstr vmx_spec_ctrl_restore_host(struct vcpu_vmx *vmx,
- unsigned int flags)
-{
- u64 hostval = this_cpu_read(x86_spec_ctrl_current);
-
- if (!cpu_feature_enabled(X86_FEATURE_MSR_SPEC_CTRL))
- return;
-
- if (flags & KVM_ENTER_SAVE_SPEC_CTRL)
- vmx->spec_ctrl = native_rdmsrq(MSR_IA32_SPEC_CTRL);
-
- /*
- * If the guest/host SPEC_CTRL values differ, restore the host value.
- *
- * For legacy IBRS, the IBRS bit always needs to be written after
- * transitioning from a less privileged predictor mode, regardless of
- * whether the guest/host values differ.
- */
- if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) ||
- vmx->spec_ctrl != hostval)
- native_wrmsrq(MSR_IA32_SPEC_CTRL, hostval);
-
- barrier_nospec();
-}
-
static fastpath_t vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu,
bool force_immediate_exit)
{
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index be6a1dc2f69f..f62007a5c2a7 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -367,7 +367,6 @@ void vmx_set_virtual_apic_mode(struct kvm_vcpu *vcpu);
struct vmx_uret_msr *vmx_find_uret_msr(struct vcpu_vmx *vmx, u32 msr);
void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu);
void vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp);
-void vmx_spec_ctrl_restore_host(struct vcpu_vmx *vmx, unsigned int flags);
unsigned int __vmx_vcpu_enter_flags(struct vcpu_vmx *vmx);
bool __vmx_vcpu_run(struct vcpu_vmx *vmx, unsigned int flags);
void vmx_ept_load_pdptrs(struct kvm_vcpu *vcpu);
--
2.52.0