KVM: x86: Suppress MMIO that is triggered during task switch emulation

Explicitly suppress userspace emulated MMIO exits that are triggered when
emulating a task switch as KVM doesn't support userspace MMIO during
complex (multi-step) emulation. Silently ignoring the exit request can
result in the WARN_ON_ONCE(vcpu->mmio_needed) firing if KVM exits to
userspace for some other reason prior to purging mmio_needed.

See commit 0dc902267cb3 ("KVM: x86: Suppress pending MMIO write exits if
emulator detects exception") for more details on KVM's limitations with
respect to emulated MMIO during complex emulator flows.

Reported-by: [e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>
Message-ID: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/kvm/x86.c

···
 
 	ret = emulator_task_switch(ctxt, tss_selector, idt_index, reason,
 				   has_error_code, error_code);
-	if (ret) {
+
+	/*
+	 * Report an error userspace if MMIO is needed, as KVM doesn't support
+	 * MMIO during a task switch (or any other complex operation).
+	 */
+	if (ret || vcpu->mmio_needed) {
+		vcpu->mmio_needed = false;
 		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
 		vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
 		vcpu->run->internal.ndata = 0;