Open code the differences in read vs. write userspace MMIO exits instead
of burying three lines of code behind indirect callbacks, as splitting the
logic makes it extremely hard to track that KVM's handling of reads vs.
write is _significantly_ different.  Add a comment to explain why the
semantics are different, and how on earth an MMIO write ends up triggering
an exit to userspace.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 33 +++++++++++++--------------------
 1 file changed, 13 insertions(+), 20 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7cbd6f7d8578..5fde5bb010e7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8116,8 +8116,6 @@ struct read_write_emulator_ops {
 				  void *val, int bytes);
 	int (*read_write_mmio)(struct kvm_vcpu *vcpu, gpa_t gpa,
 			       int bytes, void *val);
-	int (*read_write_exit_mmio)(struct kvm_vcpu *vcpu, gpa_t gpa,
-				    void *val, int bytes);
 	bool write;
 };
 
@@ -8139,31 +8137,14 @@ static int write_mmio(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes, void *val)
 	return vcpu_mmio_write(vcpu, gpa, bytes, val);
 }
 
-static int read_exit_mmio(struct kvm_vcpu *vcpu, gpa_t gpa,
-			  void *val, int bytes)
-{
-	return X86EMUL_IO_NEEDED;
-}
-
-static int write_exit_mmio(struct kvm_vcpu *vcpu, gpa_t gpa,
-			   void *val, int bytes)
-{
-	struct kvm_mmio_fragment *frag = &vcpu->mmio_fragments[0];
-
-	memcpy(vcpu->run->mmio.data, frag->data, min(8u, frag->len));
-	return X86EMUL_CONTINUE;
-}
-
 static const struct read_write_emulator_ops read_emultor = {
 	.read_write_emulate = read_emulate,
 	.read_write_mmio = vcpu_mmio_read,
-	.read_write_exit_mmio = read_exit_mmio,
 };
 
 static const struct read_write_emulator_ops write_emultor = {
 	.read_write_emulate = write_emulate,
 	.read_write_mmio = write_mmio,
-	.read_write_exit_mmio = write_exit_mmio,
 	.write = true,
 };
 
@@ -8296,7 +8277,19 @@ static int emulator_read_write(struct x86_emulate_ctxt *ctxt,
 	vcpu->run->exit_reason = KVM_EXIT_MMIO;
 	vcpu->run->mmio.phys_addr = frag->gpa;
 
-	return ops->read_write_exit_mmio(vcpu, frag->gpa, val, bytes);
+	/*
+	 * For MMIO reads, stop emulating and immediately exit to userspace, as
+	 * KVM needs the value to correctly emulate the instruction.  For MMIO
+	 * writes, continue emulating as the write to MMIO is a side effect for
+	 * all intents and purposes.  KVM will still exit to userspace, but
+	 * after completing emulation (see the check on vcpu->mmio_needed in
+	 * x86_emulate_instruction()).
+	 */
+	if (!ops->write)
+		return X86EMUL_IO_NEEDED;
+
+	memcpy(vcpu->run->mmio.data, frag->data, min(8u, frag->len));
+	return X86EMUL_CONTINUE;
 }
 
 static int emulator_read_emulated(struct x86_emulate_ctxt *ctxt,
-- 
2.53.0.414.gf7e9f6c205-goog