linux-kernel - [patch V2 1/7] x86/kvm: Move context tracking where it belongs

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Date:   Wed, 08 Jul 2020 21:51:54 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>,
        kvm@...r.kernel.org,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>
Subject: [patch V2 1/7] x86/kvm: Move context tracking where it belongs

From: Thomas Gleixner <tglx@...utronix.de>

Context tracking for KVM happens way too early in the vcpu_run()
code. Anything after guest_enter_irqoff() and before guest_exit_irqoff()
cannot use RCU and should also be not instrumented.

The current way of doing this covers way too much code. Move it closer to
the actual vmenter/exit code.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@...cle.com>
Acked-by: Peter Zijlstra <peterz@...radead.org>
Acked-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/svm/svm.c |   16 ++++++++++++++++
 arch/x86/kvm/vmx/vmx.c |   10 ++++++++++
 arch/x86/kvm/x86.c     |    2 --
 3 files changed, 26 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3399,6 +3399,14 @@ static __no_kcsan fastpath_t svm_vcpu_ru
 	 */
 	x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
 
+	/*
+	 * Tell context tracking that this CPU is about to enter guest
+	 * mode. This has to be after x86_spec_ctrl_set_guest() because
+	 * that can take locks (lockdep needs RCU) and calls into world and
+	 * some more.
+	 */
+	guest_enter_irqoff();
+
 	__svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs);
 
 #ifdef CONFIG_X86_64
@@ -3409,6 +3417,14 @@ static __no_kcsan fastpath_t svm_vcpu_ru
 	loadsegment(gs, svm->host.gs);
 #endif
 #endif
+	/*
+	 * Tell context tracking that this CPU is back.
+	 *
+	 * This needs to be done before the below as native_read_msr()
+	 * contains a tracepoint and x86_spec_ctrl_restore_host() calls
+	 * into world and some more.
+	 */
+	guest_exit_irqoff();
 
 	/*
 	 * We do not use IBRS in the kernel. If this vCPU has used the
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6728,6 +6728,11 @@ static fastpath_t vmx_vcpu_run(struct kv
 	 */
 	x86_spec_ctrl_set_guest(vmx->spec_ctrl, 0);
 
+	/*
+	 * Tell context tracking that this CPU is about to enter guest mode.
+	 */
+	guest_enter_irqoff();
+
 	/* L1D Flush includes CPU buffer clear to mitigate MDS */
 	if (static_branch_unlikely(&vmx_l1d_should_flush))
 		vmx_l1d_flush(vcpu);
@@ -6743,6 +6748,11 @@ static fastpath_t vmx_vcpu_run(struct kv
 	vcpu->arch.cr2 = read_cr2();
 
 	/*
+	 * Tell context tracking that this CPU is back.
+	 */
+	guest_exit_irqoff();
+
+	/*
 	 * We do not use IBRS in the kernel. If this vCPU has used the
 	 * SPEC_CTRL MSR it may have left it on; save the value and
 	 * turn it off. This is much more efficient than blindly adding
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8501,7 +8501,6 @@ static int vcpu_enter_guest(struct kvm_v
 	}
 
 	trace_kvm_entry(vcpu->vcpu_id);
-	guest_enter_irqoff();
 
 	fpregs_assert_state_consistent();
 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
@@ -8563,7 +8562,6 @@ static int vcpu_enter_guest(struct kvm_v
 	local_irq_disable();
 	kvm_after_interrupt(vcpu);
 
-	guest_exit_irqoff();
 	if (lapic_in_kernel(vcpu)) {
 		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;
 		if (delta != S64_MIN) {