KVM: X86: Remove unneeded KVM_DEBUGREG_RELOAD

Commit ae561ed ("KVM: x86: DR0-DR3 are not clear on reset") added code to
ensure eff_db are updated when they're modified through non-standard paths.

But there is no reason to also update hardware DRs unless hardware breakpoints
are active or DR exiting is disabled, and in those cases updating hardware is
handled by KVM_DEBUGREG_WONT_EXIT and KVM_DEBUGREG_BP_ENABLED.

KVM_DEBUGREG_RELOAD just causes unnecesarry load of hardware DRs and is better
to be removed.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20210809174307.145263-1-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -523,7 +523,6 @@ struct kvm_pmu_ops;
 enum {
 	KVM_DEBUGREG_BP_ENABLED = 1,
 	KVM_DEBUGREG_WONT_EXIT = 2,
-	KVM_DEBUGREG_RELOAD = 4,
 };
 
 struct kvm_mtrr_range {

arch/x86/kvm/x86.c

@@ -1181,7 +1181,6 @@ static void kvm_update_dr0123(struct kvm_vcpu *vcpu)
 	if (!(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP)) {
 		for (i = 0; i < KVM_NR_DB_REGS; i++)
 			vcpu->arch.eff_db[i] = vcpu->arch.db[i];
-		vcpu->arch.switch_db_regs |= KVM_DEBUGREG_RELOAD;
 	}
 }
 
@@ -9604,7 +9603,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		set_debugreg(vcpu->arch.eff_db[2], 2);
 		set_debugreg(vcpu->arch.eff_db[3], 3);
 		set_debugreg(vcpu->arch.dr6, 6);
-		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
 	} else if (unlikely(hw_breakpoint_active())) {
 		set_debugreg(0, 7);
 	}
@@ -9634,7 +9632,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		static_call(kvm_x86_sync_dirty_debug_regs)(vcpu);
 		kvm_update_dr0123(vcpu);
 		kvm_update_dr7(vcpu);
-		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
 	}
 
 	/*