KVM: x86: Drop kvm_x86_ops.set_dr6() in favor of a new KVM_RUN flag

Instruct vendor code to load the guest's DR6 into hardware via a new
KVM_RUN flag, and remove kvm_x86_ops.set_dr6(), whose sole purpose was to
load vcpu->arch.dr6 into hardware when DR6 can be read/written directly
by the guest.

Note, TDX already WARNs on any run_flag being set, i.e. will yell if KVM
thinks DR6 needs to be reloaded.  TDX vCPUs force KVM_DEBUGREG_AUTO_SWITCH
and never clear the flag, i.e. should never observe KVM_RUN_LOAD_GUEST_DR6.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250610232010.162191-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/include/asm/kvm-x86-ops.h

@@ -49,7 +49,6 @@ KVM_X86_OP(set_idt)
 KVM_X86_OP(get_gdt)
 KVM_X86_OP(set_gdt)
 KVM_X86_OP(sync_dirty_debug_regs)
-KVM_X86_OP(set_dr6)
 KVM_X86_OP(set_dr7)
 KVM_X86_OP(cache_reg)
 KVM_X86_OP(get_rflags)

arch/x86/include/asm/kvm_host.h

@@ -1676,6 +1676,7 @@ static inline u16 kvm_lapic_irq_dest_mode(bool dest_mode_logical)
 
 enum kvm_x86_run_flags {
 	KVM_RUN_FORCE_IMMEDIATE_EXIT	= BIT(0),
+	KVM_RUN_LOAD_GUEST_DR6		= BIT(1),
 };
 
 struct kvm_x86_ops {
@@ -1728,7 +1729,6 @@ struct kvm_x86_ops {
 	void (*get_gdt)(struct kvm_vcpu *vcpu, struct desc_ptr *dt);
 	void (*set_gdt)(struct kvm_vcpu *vcpu, struct desc_ptr *dt);
 	void (*sync_dirty_debug_regs)(struct kvm_vcpu *vcpu);
-	void (*set_dr6)(struct kvm_vcpu *vcpu, unsigned long value);
 	void (*set_dr7)(struct kvm_vcpu *vcpu, unsigned long value);
 	void (*cache_reg)(struct kvm_vcpu *vcpu, enum kvm_reg reg);
 	unsigned long (*get_rflags)(struct kvm_vcpu *vcpu);

arch/x86/kvm/svm/svm.c

@@ -4438,10 +4438,13 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
 	svm_hv_update_vp_id(svm->vmcb, vcpu);
 
 	/*
-	 * Run with all-zero DR6 unless needed, so that we can get the exact cause
+	 * Run with all-zero DR6 unless the guest can write DR6 freely, so that
-	 * of a #DB.
+	 * KVM can get the exact cause of a #DB.  Note, loading guest DR6 from
+	 * KVM's snapshot is only necessary when DR accesses won't exit.
 	 */
-	if (likely(!(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)))
+	if (unlikely(run_flags & KVM_RUN_LOAD_GUEST_DR6))
+		svm_set_dr6(vcpu, vcpu->arch.dr6);
+	else if (likely(!(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)))
 		svm_set_dr6(vcpu, DR6_ACTIVE_LOW);
 
 	clgi();
@@ -5252,7 +5255,6 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
 	.set_idt = svm_set_idt,
 	.get_gdt = svm_get_gdt,
 	.set_gdt = svm_set_gdt,
-	.set_dr6 = svm_set_dr6,
 	.set_dr7 = svm_set_dr7,
 	.sync_dirty_debug_regs = svm_sync_dirty_debug_regs,
 	.cache_reg = svm_cache_reg,

arch/x86/kvm/vmx/main.c

@@ -489,14 +489,6 @@ static void vt_set_gdt(struct kvm_vcpu *vcpu, struct desc_ptr *dt)
 	vmx_set_gdt(vcpu, dt);
 }
 
-static void vt_set_dr6(struct kvm_vcpu *vcpu, unsigned long val)
-{
-	if (is_td_vcpu(vcpu))
-		return;
-
-	vmx_set_dr6(vcpu, val);
-}
-
 static void vt_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
 {
 	if (is_td_vcpu(vcpu))
@@ -943,7 +935,6 @@ struct kvm_x86_ops vt_x86_ops __initdata = {
 	.set_idt = vt_op(set_idt),
 	.get_gdt = vt_op(get_gdt),
 	.set_gdt = vt_op(set_gdt),
-	.set_dr6 = vt_op(set_dr6),
 	.set_dr7 = vt_op(set_dr7),
 	.sync_dirty_debug_regs = vt_op(sync_dirty_debug_regs),
 	.cache_reg = vt_op(cache_reg),

arch/x86/kvm/vmx/vmx.c

@@ -5606,12 +5606,6 @@ void vmx_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)
 	set_debugreg(DR6_RESERVED, 6);
 }
 
-void vmx_set_dr6(struct kvm_vcpu *vcpu, unsigned long val)
-{
-	lockdep_assert_irqs_disabled();
-	set_debugreg(vcpu->arch.dr6, 6);
-}
-
 void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
 {
 	vmcs_writel(GUEST_DR7, val);
@@ -7370,6 +7364,9 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
 		vmcs_writel(GUEST_RIP, vcpu->arch.regs[VCPU_REGS_RIP]);
 	vcpu->arch.regs_dirty = 0;
 
+	if (run_flags & KVM_RUN_LOAD_GUEST_DR6)
+		set_debugreg(vcpu->arch.dr6, 6);
+
 	/*
 	 * Refresh vmcs.HOST_CR3 if necessary.  This must be done immediately
 	 * prior to VM-Enter, as the kernel may load a new ASID (PCID) any time

arch/x86/kvm/x86.c

@@ -11046,7 +11046,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		set_debugreg(vcpu->arch.eff_db[3], 3);
 		/* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */
 		if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
-			kvm_x86_call(set_dr6)(vcpu, vcpu->arch.dr6);
+			run_flags |= KVM_RUN_LOAD_GUEST_DR6;
 	} else if (unlikely(hw_breakpoint_active())) {
 		set_debugreg(0, 7);
 	}