[PATCH v4 10/10] x86/tdx: Handle MWAIT and MONITOR

From: Kuppuswamy Sathyanarayanan
Date: Wed Aug 04 2021 - 15:19:55 EST

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.

You can find the list of unsupported instructions in Intel
Trust Domain Extensions (Intel® TDX) Module specification,
sec 9.2.2 and in Guest-Host Communication Interface (GHCI)
Specification for Intel TDX, sec 2.4.1.

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 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 ab4f7c962ddd..4080f7546e62 100644
--- a/arch/x86/kernel/tdx.c
+++ b/arch/x86/kernel/tdx.c
@@ -360,6 +360,14 @@ int tdg_handle_virtualization_exception(struct pt_regs *regs,
return -EFAULT;
+ /*
+ * 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;
pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
return -EFAULT;