KVM: x86: Always complete hypercall via function callback

Finish "emulation" of KVM hypercalls by function callback, even when the
hypercall is handled entirely within KVM, i.e. doesn't require an exit to
userspace, and refactor __kvm_emulate_hypercall()'s return value to *only*
communicate whether or not KVM should exit to userspace or resume the
guest.

(Ab)Use vcpu->run->hypercall.ret to propagate the return value to the
callback, purely to avoid having to add a trampoline for every completion
callback.

Using the function return value for KVM's control flow eliminates the
multiplexed return value, where '0' for KVM_HC_MAP_GPA_RANGE (and only
that hypercall) means "exit to userspace".

Note, the unnecessary extra indirect call and thus potential retpoline
will be eliminated in the near future by converting the intermediate layer
to a macro.

Suggested-by: Binbin Wu <binbin.wu@linux.intel.com>
Suggested-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Message-ID: <20241128004344.4072099-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 17 insertions, 21 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 754d8166eabf..53ef7fa7094b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9982,10 +9982,11 @@ static int complete_hypercall_exit(struct kvm_vcpu *vcpu)
 	return kvm_skip_emulated_instruction(vcpu);
 }
 
-unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
-				      unsigned long a0, unsigned long a1,
-				      unsigned long a2, unsigned long a3,
-				      int op_64_bit, int cpl)
+int __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
+			    unsigned long a0, unsigned long a1,
+			    unsigned long a2, unsigned long a3,
+			    int op_64_bit, int cpl,
+			    int (*complete_hypercall)(struct kvm_vcpu *))
 {
 	unsigned long ret;
 
@@ -10068,7 +10069,7 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
 			vcpu->run->hypercall.flags |= KVM_EXIT_HYPERCALL_LONG_MODE;
 
 		WARN_ON_ONCE(vcpu->run->hypercall.flags & KVM_EXIT_HYPERCALL_MBZ);
-		vcpu->arch.complete_userspace_io = complete_hypercall_exit;
+		vcpu->arch.complete_userspace_io = complete_hypercall;
 		return 0;
 	}
 	default:
@@ -10077,13 +10078,14 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
 	}
 
 out:
-	return ret;
+	vcpu->run->hypercall.ret = ret;
+	return complete_hypercall(vcpu);
 }
 EXPORT_SYMBOL_GPL(__kvm_emulate_hypercall);
 
 int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 {
-	unsigned long nr, a0, a1, a2, a3, ret;
+	unsigned long nr, a0, a1, a2, a3;
 	int op_64_bit;
 	int cpl;
 
@@ -10101,16 +10103,8 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 	op_64_bit = is_64_bit_hypercall(vcpu);
 	cpl = kvm_x86_call(get_cpl)(vcpu);
 
-	ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl);
-	if (nr == KVM_HC_MAP_GPA_RANGE && !ret)
-		/* MAP_GPA tosses the request to the user space. */
-		return 0;
-
-	if (!op_64_bit)
-		ret = (u32)ret;
-	kvm_rax_write(vcpu, ret);
-
-	return kvm_skip_emulated_instruction(vcpu);
+	return __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl,
+				       complete_hypercall_exit);
 }
 EXPORT_SYMBOL_GPL(kvm_emulate_hypercall);
 
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 6db13b696468..28adc8ea04bf 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -617,10 +617,12 @@ static inline bool user_exit_on_hypercall(struct kvm *kvm, unsigned long hc_nr)
 	return kvm->arch.hypercall_exit_enabled & BIT(hc_nr);
 }
 
-unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
-				      unsigned long a0, unsigned long a1,
-				      unsigned long a2, unsigned long a3,
-				      int op_64_bit, int cpl);
+int __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
+			    unsigned long a0, unsigned long a1,
+			    unsigned long a2, unsigned long a3,
+			    int op_64_bit, int cpl,
+			    int (*complete_hypercall)(struct kvm_vcpu *));
+
 int kvm_emulate_hypercall(struct kvm_vcpu *vcpu);
 
 #endif