linux-kernel - [PATCH 02/11] KVM: VMX: Handle APIC-write offset wrangling in VMX code

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Message-Id: <20220204214205.3306634-3-seanjc@google.com>
Date:   Fri,  4 Feb 2022 21:41:56 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.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, Zeng Guang <guang.zeng@...el.com>,
        Chao Gao <chao.gao@...el.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH 02/11] KVM: VMX: Handle APIC-write offset wrangling in VMX code

Move the vAPIC offset adjustments done in the APIC-write trap path from
common x86 to VMX in anticipation of using the nodecode path for SVM's
AVIC.  The adjustment reflects hardware behavior, i.e. it's technically a
property of VMX, no common x86.  SVM's AVIC behavior is identical, so
it's a bit of a moot point, the goal is purely to make it easier to
understand why the adjustment is ok.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/lapic.c   |  3 ---
 arch/x86/kvm/vmx/vmx.c | 11 +++++++++--
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 4662469240bc..fbce455a9d17 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2188,9 +2188,6 @@ void kvm_apic_write_nodecode(struct kvm_vcpu *vcpu, u32 offset)
 {
 	u32 val = 0;
 
-	/* hw has done the conditional check and inst decode */
-	offset &= 0xff0;
-
 	kvm_lapic_reg_read(vcpu->arch.apic, offset, 4, &val);
 
 	/* TODO: optimize to just emulate side effect w/o one more write */
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b1165bb13a5a..1b135473677b 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5302,9 +5302,16 @@ static int handle_apic_eoi_induced(struct kvm_vcpu *vcpu)
 static int handle_apic_write(struct kvm_vcpu *vcpu)
 {
 	unsigned long exit_qualification = vmx_get_exit_qual(vcpu);
-	u32 offset = exit_qualification & 0xfff;
 
-	/* APIC-write VM exit is trap-like and thus no need to adjust IP */
+	/*
+	 * APIC-write VM-Exit is trap-like, KVM doesn't need to advance RIP and
+	 * hardware has done any necessary aliasing, offset adjustments, etc...
+	 * for the access.  I.e. the correct value has already been  written to
+	 * the vAPIC page for the correct 16-byte chunk.  KVM needs only to
+	 * retrieve the register value and emulate the access.
+	 */
+	u32 offset = exit_qualification & 0xff0;
+
 	kvm_apic_write_nodecode(vcpu, offset);
 	return 1;
 }
-- 
2.35.0.263.gb82422642f-goog