[PATCH v12 20/22] x86/virt/tdx: Allow SEAMCALL to handle #UD and #GP

[Kai Huang <kai.huang@intel.com>]

On the platform with the "partial write machine check" erratum, a kernel
partial write to TDX private memory may cause unexpected machine check.
It would be nice if the #MC handler could print additional information
to show the #MC was TDX private memory error due to possible kernel bug.

To do that, the machine check handler needs to use SEAMCALL to query
page type of the error memory from the TDX module, because there's no
existing infrastructure to track TDX private pages.

SEAMCALL instruction causes #UD if CPU isn't in VMX operation.  In #MC
handler, it is legal that CPU isn't in VMX operation when making this
SEAMCALL.  Extend the TDX_MODULE_CALL macro to handle #UD so the
SEAMCALL can return error code instead of Oops in the #MC handler.
Opportunistically handles #GP too since they share the same code.

A bonus is when kernel mistakenly calls SEAMCALL when CPU isn't in VMX
operation, or when TDX isn't enabled by the BIOS, or when the BIOS is
buggy, the kernel can get a nicer error message rather than a less
understandable Oops.

Signed-off-by: Kai Huang <kai.huang@intel.com>
---

v11 -> v12 (new patch):
  - Splitted out from "SEAMCALL infrastructure" patch for better review.
  - Provide justification in changelog (Dave/David)

---
 arch/x86/include/asm/tdx.h      |  5 +++++
 arch/x86/virt/vmx/tdx/tdx.c     |  7 +++++++
 arch/x86/virt/vmx/tdx/tdxcall.S | 19 +++++++++++++++++--
 3 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index e95c9fbf52e4..8d3f85bcccc1 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -8,6 +8,8 @@
 #include <asm/ptrace.h>
 #include <asm/shared/tdx.h>
 
+#include <asm/trapnr.h>
+
 /*
  * SW-defined error codes.
  *
@@ -18,6 +20,9 @@
 #define TDX_SW_ERROR			(TDX_ERROR | GENMASK_ULL(47, 40))
 #define TDX_SEAMCALL_VMFAILINVALID	(TDX_SW_ERROR | _UL(0xFFFF0000))
 
+#define TDX_SEAMCALL_GP			(TDX_SW_ERROR | X86_TRAP_GP)
+#define TDX_SEAMCALL_UD			(TDX_SW_ERROR | X86_TRAP_UD)
+
 #ifndef __ASSEMBLY__
 
 /* TDX supported page sizes from the TDX module ABI. */
diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 1107f4227568..eba7ff91206d 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -93,6 +93,13 @@ static int __always_unused seamcall(u64 fn, u64 rcx, u64 rdx, u64 r8, u64 r9,
 	case TDX_SEAMCALL_VMFAILINVALID:
 		pr_err_once("module is not loaded.\n");
 		return -ENODEV;
+	case TDX_SEAMCALL_GP:
+		pr_err_once("not enabled by BIOS.\n");
+		return -ENODEV;
+	case TDX_SEAMCALL_UD:
+		pr_err_once("SEAMCALL failed: CPU %d is not in VMX operation.\n",
+				cpu);
+		return -EINVAL;
 	default:
 		pr_err_once("SEAMCALL failed: CPU %d: leaf %llu, error 0x%llx.\n",
 				cpu, fn, sret);
diff --git a/arch/x86/virt/vmx/tdx/tdxcall.S b/arch/x86/virt/vmx/tdx/tdxcall.S
index 49a54356ae99..757b0c34be10 100644
--- a/arch/x86/virt/vmx/tdx/tdxcall.S
+++ b/arch/x86/virt/vmx/tdx/tdxcall.S
@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 #include <asm/asm-offsets.h>
 #include <asm/tdx.h>
+#include <asm/asm.h>
 
 /*
  * TDCALL and SEAMCALL are supported in Binutils >= 2.36.
@@ -45,6 +46,7 @@
 	/* Leave input param 2 in RDX */
 
 	.if \host
+1:
 	seamcall
 	/*
 	 * SEAMCALL instruction is essentially a VMExit from VMX root
@@ -57,10 +59,23 @@
 	 * This value will never be used as actual SEAMCALL error code as
 	 * it is from the Reserved status code class.
 	 */
-	jnc .Lno_vmfailinvalid
+	jnc .Lseamcall_out
 	mov $TDX_SEAMCALL_VMFAILINVALID, %rax
-.Lno_vmfailinvalid:
+	jmp .Lseamcall_out
+2:
+	/*
+	 * SEAMCALL caused #GP or #UD.  By reaching here %eax contains
+	 * the trap number.  Convert the trap number to the TDX error
+	 * code by setting TDX_SW_ERROR to the high 32-bits of %rax.
+	 *
+	 * Note cannot OR TDX_SW_ERROR directly to %rax as OR instruction
+	 * only accepts 32-bit immediate at most.
+	 */
+	mov $TDX_SW_ERROR, %r12
+	orq %r12, %rax
 
+	_ASM_EXTABLE_FAULT(1b, 2b)
+.Lseamcall_out:
 	.else
 	tdcall
 	.endif
-- 
2.40.1