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Message-ID: <20251016200417.97003-5-seanjc@google.com>
Date: Thu, 16 Oct 2025 13:04:17 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Brendan Jackman <jackmanb@...gle.com>
Subject: [PATCH v3 4/4] KVM: x86: Unify L1TF flushing under per-CPU variable
From: Brendan Jackman <jackmanb@...gle.com>
Currently the tracking of the need to flush L1D for L1TF is tracked by
two bits: one per-CPU and one per-vCPU.
The per-vCPU bit is always set when the vCPU shows up on a core, so
there is no interesting state that's truly per-vCPU. Indeed, this is a
requirement, since L1D is a part of the physical CPU.
So simplify this by combining the two bits.
The vCPU bit was being written from preemption-enabled regions. To play
nice with those cases, wrap all calls from KVM and use a raw write so that
request a flush with preemption enabled doesn't trigger what would
effectively be DEBUG_PREEMPT false positives. Preemption doesn't need to
be disabled, as kvm_arch_vcpu_load() will mark the new CPU as needing a
flush if the vCPU task is migrated, or if userspace runs the vCPU on a
different task.
Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
[sean: put raw write in KVM instead of in a hardirq.h variant]
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
arch/x86/include/asm/kvm_host.h | 3 ---
arch/x86/kvm/mmu/mmu.c | 2 +-
arch/x86/kvm/vmx/nested.c | 2 +-
arch/x86/kvm/vmx/vmx.c | 20 +++++---------------
arch/x86/kvm/x86.c | 6 +++---
arch/x86/kvm/x86.h | 14 ++++++++++++++
6 files changed, 24 insertions(+), 23 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 48598d017d6f..fcdc65ab13d8 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1055,9 +1055,6 @@ struct kvm_vcpu_arch {
/* be preempted when it's in kernel-mode(cpl=0) */
bool preempted_in_kernel;
- /* Flush the L1 Data cache for L1TF mitigation on VMENTER */
- bool l1tf_flush_l1d;
-
/* Host CPU on which VM-entry was most recently attempted */
int last_vmentry_cpu;
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 18d69d48bc55..4e016582adc7 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4859,7 +4859,7 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
*/
BUILD_BUG_ON(lower_32_bits(PFERR_SYNTHETIC_MASK));
- vcpu->arch.l1tf_flush_l1d = true;
+ kvm_request_l1tf_flush_l1d();
if (!flags) {
trace_kvm_page_fault(vcpu, fault_address, error_code);
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 3fca63a261f5..468a013d9ef3 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3880,7 +3880,7 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
goto vmentry_failed;
/* Hide L1D cache contents from the nested guest. */
- vcpu->arch.l1tf_flush_l1d = true;
+ kvm_request_l1tf_flush_l1d();
/*
* Must happen outside of nested_vmx_enter_non_root_mode() as it will
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index e91d99211efe..0347d321a86e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -395,26 +395,16 @@ static noinstr bool vmx_l1d_flush(struct kvm_vcpu *vcpu)
* 'always'
*/
if (static_branch_likely(&vmx_l1d_flush_cond)) {
- bool flush_l1d;
-
/*
- * Clear the per-vcpu flush bit, it gets set again if the vCPU
+ * Clear the per-cpu flush bit, it gets set again if the vCPU
* is reloaded, i.e. if the vCPU is scheduled out or if KVM
* exits to userspace, or if KVM reaches one of the unsafe
- * VMEXIT handlers, e.g. if KVM calls into the emulator.
+ * VMEXIT handlers, e.g. if KVM calls into the emulator,
+ * or from the interrupt handlers.
*/
- flush_l1d = vcpu->arch.l1tf_flush_l1d;
- vcpu->arch.l1tf_flush_l1d = false;
-
- /*
- * Clear the per-cpu flush bit, it gets set again from
- * the interrupt handlers.
- */
- flush_l1d |= kvm_get_cpu_l1tf_flush_l1d();
+ if (!kvm_get_cpu_l1tf_flush_l1d())
+ return;
kvm_clear_cpu_l1tf_flush_l1d();
-
- if (!flush_l1d)
- return false;
}
vcpu->stat.l1d_flush++;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b4b5d2d09634..851f078cd5ca 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5189,7 +5189,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
{
struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
- vcpu->arch.l1tf_flush_l1d = true;
+ kvm_request_l1tf_flush_l1d();
if (vcpu->scheduled_out && pmu->version && pmu->event_count) {
pmu->need_cleanup = true;
@@ -7999,7 +7999,7 @@ int kvm_write_guest_virt_system(struct kvm_vcpu *vcpu, gva_t addr, void *val,
unsigned int bytes, struct x86_exception *exception)
{
/* kvm_write_guest_virt_system can pull in tons of pages. */
- vcpu->arch.l1tf_flush_l1d = true;
+ kvm_request_l1tf_flush_l1d();
return kvm_write_guest_virt_helper(addr, val, bytes, vcpu,
PFERR_WRITE_MASK, exception);
@@ -9395,7 +9395,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
return handle_emulation_failure(vcpu, emulation_type);
}
- vcpu->arch.l1tf_flush_l1d = true;
+ kvm_request_l1tf_flush_l1d();
if (!(emulation_type & EMULTYPE_NO_DECODE)) {
kvm_clear_exception_queue(vcpu);
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index f3dc77f006f9..cd67ccbb747f 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -420,6 +420,20 @@ static inline bool kvm_check_has_quirk(struct kvm *kvm, u64 quirk)
return !(kvm->arch.disabled_quirks & quirk);
}
+static __always_inline void kvm_request_l1tf_flush_l1d(void)
+{
+#if IS_ENABLED(CONFIG_CPU_MITIGATIONS) && IS_ENABLED(CONFIG_KVM_INTEL)
+ /*
+ * Use a raw write to set the per-CPU flag, as KVM will ensure a flush
+ * even if preemption is currently enabled.. If the current vCPU task
+ * is migrated to a different CPU (or userspace runs the vCPU on a
+ * different task) before the next VM-Entry, then kvm_arch_vcpu_load()
+ * will request a flush on the new CPU.
+ */
+ raw_cpu_write(irq_stat.kvm_cpu_l1tf_flush_l1d, 1);
+#endif
+}
+
void kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip);
u64 get_kvmclock_ns(struct kvm *kvm);
--
2.51.0.858.gf9c4a03a3a-goog
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