linux-kernel - [PATCH 2/2] KVM: x86: Skip zeroing of MPX state on reset event

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Date:   Mon,  9 Dec 2019 12:19:32 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] KVM: x86: Skip zeroing of MPX state on reset event

Don't bother zeroing out MPX state in the guest's FPU on a reset event,
the guest's FPU is always zero allocated and there is no path between
kvm_arch_vcpu_create() and kvm_arch_vcpu_setup() that can lead to guest
MPX state being modified.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
 arch/x86/kvm/x86.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 854ae27bb021..e6f4174f55cd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9194,15 +9194,14 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 	kvm_async_pf_hash_reset(vcpu);
 	vcpu->arch.apf.halted = false;
 
-	if (kvm_mpx_supported()) {
+	if (kvm_mpx_supported() && init_event) {
 		void *mpx_state_buffer;
 
 		/*
-		 * To avoid have the INIT path from kvm_apic_has_events() that be
-		 * called with loaded FPU and does not let userspace fix the state.
+		 * Temporarily flush the guest's FPU to memory so that zeroing
+		 * out the MPX areas is done using up-to-date state.
 		 */
-		if (init_event)
-			kvm_put_guest_fpu(vcpu);
+		kvm_put_guest_fpu(vcpu);
 		mpx_state_buffer = get_xsave_addr(&vcpu->arch.guest_fpu->state.xsave,
 					XFEATURE_BNDREGS);
 		if (mpx_state_buffer)
@@ -9211,8 +9210,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 					XFEATURE_BNDCSR);
 		if (mpx_state_buffer)
 			memset(mpx_state_buffer, 0, sizeof(struct mpx_bndcsr));
-		if (init_event)
-			kvm_load_guest_fpu(vcpu);
+		kvm_load_guest_fpu(vcpu);
 	}
 
 	if (!init_event) {
-- 
2.24.0