[PATCH v4 02/16] KVM: x86: Set vCPU exit reason to KVM_EXIT_UNKNOWN at the start of KVM_RUN

[Anish Moorthy <amoorthy@google.com>]

Give kvm_run.exit_reason a defined initial value on entry into KVM_RUN:
other architectures (riscv, arm64) already use KVM_EXIT_UNKNOWN for this
purpose, so copy that convention.

This gives vCPUs trying to fill the run struct a mechanism to avoid
overwriting already-populated data, albeit an imperfect one. Being able
to detect an already-populated KVM run struct will prevent at least some
bugs in the upcoming implementation of KVM_CAP_MEMORY_FAULT_INFO, which
will attempt to fill the run struct whenever a vCPU fails a guest memory
access.

Without the already-populated check, KVM_CAP_MEMORY_FAULT_INFO could
change kvm_run in any code paths which

1. Populate kvm_run for some exit and prepare to return to userspace
2. Access guest memory for some reason (but without returning -EFAULTs
    to userspace)
3. Finish the return to userspace set up in (1), now with the contents
    of kvm_run changed to contain efault info.

Signed-off-by: Anish Moorthy <amoorthy@google.com>
---
 arch/x86/kvm/x86.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ceb7c5e9cf9e..a7725d41570a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11163,6 +11163,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 	if (r <= 0)
 		goto out;
 
+	kvm_run->exit_reason = KVM_EXIT_UNKNOWN;
 	r = vcpu_run(vcpu);
 
 out:
-- 
2.41.0.rc0.172.g3f132b7071-goog