[PATCH v7 10/10] x86/tdx: Handle MWAIT and MONITOR

[Kuppuswamy Sathyanarayanan]

When running as a TDX guest, there are a number of existing, privileged
instructions that do not work. If the guest kernel uses these
instructions, the hardware generates a #VE.

List of unsupported instructions can be found in Intel Trust Domain
Extensions (Intel® TDX) Module specification, sec titled "Instructions
that Cause a #VE Unconditionally" and in Guest-Host Communication
Interface (GHCI) Specification for Intel TDX, sec titled "#VE Injected
due to disallowed instructions".

To prevent TD guests from using MWAIT/MONITOR instructions, the CPUID
flags for these instructions are already disabled by the TDX module. 
   
After the above mentioned preventive measures, if TD guests still
execute these instructions, add appropriate warning message
(WARN_ONCE()) in #VE handler. This handling behavior is same as KVM
(which also treats MWAIT/MONITOR as nops with warning once in
unsupported platforms).

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---

Changes since v6:
 * Added section title to spec reference in commit log.

Changes since v5:
 * None

Changes since v4:
 * Removed usage of We/You in commit log and comments.

Changes since v3:
 * None

Changes since v2:
 * None

 arch/x86/kernel/tdx.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
index 851ad143da03..a66520405109 100644
--- a/arch/x86/kernel/tdx.c
+++ b/arch/x86/kernel/tdx.c
@@ -364,6 +364,14 @@ int tdx_handle_virtualization_exception(struct pt_regs *regs,
 			return -EFAULT;
 		}
 		break;
+	case EXIT_REASON_MONITOR_INSTRUCTION:
+	case EXIT_REASON_MWAIT_INSTRUCTION:
+		/*
+		 * Something in the kernel used MONITOR or MWAIT despite
+		 * X86_FEATURE_MWAIT being cleared for TDX guests.
+		 */
+		WARN_ONCE(1, "TD Guest used unsupported MWAIT/MONITOR instruction\n");
+		break;
 	default:
 		pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
 		return -EFAULT;
-- 
2.25.1