[PATCH v2] KVM: TDX: Allow userspace to return errors to guest for MAPGPA

[Sagi Shahar <sagis@google.com>]

From: Vishal Annapurve <vannapurve@google.com>

MAPGPA request from TDX VMs gets split into chunks by KVM using a loop
of userspace exits until the complete range is handled.

In some cases userspace VMM might decide to break the MAPGPA operation
and continue it later. For example: in the case of intrahost migration
userspace might decide to continue the MAPGPA operation after the
migration is completed.

Allow userspace to signal to TDX guests that the MAPGPA operation should
be retried the next time the guest is scheduled.

This is potentially a breaking change since if userspace sets
hypercall.ret to a value other than EBUSY or EINVAL an EINVAL error code
will be returned to userspace. As of now QEMU never sets hypercall.ret
to a non-zero value after handling KVM_EXIT_HYPERCALL so this change
should be safe.

Signed-off-by: Vishal Annapurve <vannapurve@google.com>
Co-developed-by: Sagi Shahar <sagis@google.com>
Signed-off-by: Sagi Shahar <sagis@google.com>
---
 arch/x86/kvm/vmx/tdx.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 2d7a4d52ccfb..9bd4ffbdfecf 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1189,7 +1189,13 @@ static int tdx_complete_vmcall_map_gpa(struct kvm_vcpu *vcpu)
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
 
 	if (vcpu->run->hypercall.ret) {
-		tdvmcall_set_return_code(vcpu, TDVMCALL_STATUS_INVALID_OPERAND);
+		if (vcpu->run->hypercall.ret == EAGAIN)
+			tdvmcall_set_return_code(vcpu, TDVMCALL_STATUS_RETRY);
+		else if (vcpu->run->hypercall.ret == EINVAL)
+			tdvmcall_set_return_code(vcpu, TDVMCALL_STATUS_INVALID_OPERAND);
+		else
+			return -EINVAL;
+
 		tdx->vp_enter_args.r11 = tdx->map_gpa_next;
 		return 1;
 	}
-- 
2.52.0.457.g6b5491de43-goog