[PATCH v5 2/9] x86/bugs: Use VM_CLEAR_CPU_BUFFERS in VMX as well

Sean Christopherson
2025-11-13T23:37:52Z

From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>

TSA mitigation:

d8010d4ba43e ("x86/bugs: Add a Transient Scheduler Attacks mitigation")

introduced VM_CLEAR_CPU_BUFFERS for guests on AMD CPUs. Currently on Intel
CLEAR_CPU_BUFFERS is being used for guests which has a much broader scope
(kernel->user also).

Make mitigations on Intel consistent with TSA. This would help handling the
guest-only mitigations better in future.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
[sean: make CLEAR_CPU_BUF_VM mutually exclusive with the MMIO mitigation]
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kernel/cpu/bugs.c | 13 +++++++++----
arch/x86/kvm/vmx/vmenter.S | 2 +-
2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index d7fa03bf51b4..c3a26532a209 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -194,7 +194,7 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);

/*
* Controls CPU Fill buffer clear before VMenter. This is a subset of
- * X86_FEATURE_CLEAR_CPU_BUF, and should only be enabled when KVM-only
+ * X86_FEATURE_CLEAR_CPU_BUF_VM, and should only be enabled when KVM-only
* mitigation is required.
*/
DEFINE_STATIC_KEY_FALSE(cpu_buf_vm_clear);
@@ -489,8 +489,8 @@ static enum rfds_mitigations rfds_mitigation __ro_after_init =
IS_ENABLED(CONFIG_MITIGATION_RFDS) ? RFDS_MITIGATION_AUTO : RFDS_MITIGATION_OFF;

/*
- * Set if any of MDS/TAA/MMIO/RFDS are going to enable VERW clearing
- * through X86_FEATURE_CLEAR_CPU_BUF on kernel and guest entry.
+ * Set if any of MDS/TAA/MMIO/RFDS are going to enable VERW clearing on exit to
+ * userspace *and* on entry to KVM guests.
*/
static bool verw_clear_cpu_buf_mitigation_selected __ro_after_init;

@@ -536,6 +536,7 @@ static void __init mds_apply_mitigation(void)
if (mds_mitigation == MDS_MITIGATION_FULL ||
mds_mitigation == MDS_MITIGATION_VMWERV) {
setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+ setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF_VM);
if (!boot_cpu_has(X86_BUG_MSBDS_ONLY) &&
(mds_nosmt || smt_mitigations == SMT_MITIGATIONS_ON))
cpu_smt_disable(false);
@@ -647,6 +648,7 @@ static void __init taa_apply_mitigation(void)
* present on host, enable the mitigation for UCODE_NEEDED as well.
*/
setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+ setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF_VM);

if (taa_nosmt || smt_mitigations == SMT_MITIGATIONS_ON)
cpu_smt_disable(false);
@@ -748,6 +750,7 @@ static void __init mmio_apply_mitigation(void)
*/
if (verw_clear_cpu_buf_mitigation_selected) {
setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+ setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF_VM);
static_branch_disable(&cpu_buf_vm_clear);
} else {
static_branch_enable(&cpu_buf_vm_clear);
@@ -839,8 +842,10 @@ static void __init rfds_update_mitigation(void)

static void __init rfds_apply_mitigation(void)
{
- if (rfds_mitigation == RFDS_MITIGATION_VERW)
+ if (rfds_mitigation == RFDS_MITIGATION_VERW) {
setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+ setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF_VM);
+ }
}

static __init int rfds_parse_cmdline(char *str)
diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 93cf2ca7919a..7e7bb9b7162f 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -165,7 +165,7 @@ SYM_FUNC_START(__vmx_vcpu_run)
mov VCPU_RAX(%_ASM_AX), %_ASM_AX

/* Clobbers EFLAGS.ZF */
- CLEAR_CPU_BUFFERS
+ VM_CLEAR_CPU_BUFFERS

/* Check @flags to see if vmlaunch or vmresume is needed. */
testl $VMX_RUN_VMRESUME, WORD_SIZE(%_ASM_SP)
--
2.52.0.rc1.455.g30608eb744-goog