x86/tdx: Use ReportFatalError to report missing SEPT_VE_DISABLE

Linux TDX guests require that the SEPT_VE_DISABLE "attribute" be set.
If it is not set, the kernel is theoretically required to handle
exceptions anywhere that kernel memory is accessed, including places
like NMI handlers and in the syscall entry gap.

Rather than even try to handle these exceptions, the kernel refuses to
run if SEPT_VE_DISABLE is unset.

However, the SEPT_VE_DISABLE detection and refusal code happens very
early in boot, even before earlyprintk runs.  Calling panic() will
effectively just hang the system.

Instead, call a TDX-specific panic() function.  This makes a very simple
TDVMCALL which gets a short error string out to the hypervisor without
any console infrastructure.

Use TDG.VP.VMCALL<ReportFatalError> to report the error. The hypercall
can encode message up to 64 bytes in eight registers.

[ dhansen: tweak comment and remove while loop brackets. ]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230126221159.8635-6-kirill.shutemov%40linux.intel.com

arch/x86/coco/tdx/tdx.c

@@ -22,6 +22,7 @@
 
 /* TDX hypercall Leaf IDs */
 #define TDVMCALL_MAP_GPA		0x10001
+#define TDVMCALL_REPORT_FATAL_ERROR	0x10003
 
 /* MMIO direction */
 #define EPT_READ	0
@@ -140,6 +141,41 @@ int tdx_mcall_get_report0(u8 *reportdata, u8 *tdreport)
 }
 EXPORT_SYMBOL_GPL(tdx_mcall_get_report0);
 
+static void __noreturn tdx_panic(const char *msg)
+{
+	struct tdx_hypercall_args args = {
+		.r10 = TDX_HYPERCALL_STANDARD,
+		.r11 = TDVMCALL_REPORT_FATAL_ERROR,
+		.r12 = 0, /* Error code: 0 is Panic */
+	};
+	union {
+		/* Define register order according to the GHCI */
+		struct { u64 r14, r15, rbx, rdi, rsi, r8, r9, rdx; };
+
+		char str[64];
+	} message;
+
+	/* VMM assumes '\0' in byte 65, if the message took all 64 bytes */
+	strncpy(message.str, msg, 64);
+
+	args.r8  = message.r8;
+	args.r9  = message.r9;
+	args.r14 = message.r14;
+	args.r15 = message.r15;
+	args.rdi = message.rdi;
+	args.rsi = message.rsi;
+	args.rbx = message.rbx;
+	args.rdx = message.rdx;
+
+	/*
+	 * This hypercall should never return and it is not safe
+	 * to keep the guest running. Call it forever if it
+	 * happens to return.
+	 */
+	while (1)
+		__tdx_hypercall(&args, 0);
+}
+
 static void tdx_parse_tdinfo(u64 *cc_mask)
 {
 	struct tdx_module_output out;
@@ -172,7 +208,7 @@ static void tdx_parse_tdinfo(u64 *cc_mask)
 	 */
 	td_attr = out.rdx;
 	if (!(td_attr & ATTR_SEPT_VE_DISABLE))
-		panic("TD misconfiguration: SEPT_VE_DISABLE attibute must be set.\n");
+		tdx_panic("TD misconfiguration: SEPT_VE_DISABLE attribute must be set.");
 }
 
 /*