x86/tdx-tests: Add a port of a kvm unit test

Add a ported version of one unit test from x86/debug.c from kvm-unit-tests
that fails with TDX. It's split into 3 to better illustrate what's going
on: one for stepping over instructions that are emulated via #VE, and one
each for CPUID 0 and RDMSR 0x1a0 that are both emulated by the TDX module.

The problem is that the TDX module fails to raise #DB for these
instructions, so this unit test will hopefully help debug/fix this aspect
of the TDX module.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

arch/x86/coco/tdx/Makefile

@@ -1,3 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0
 
-obj-y += tdx.o tdcall.o
+obj-y += tdx.o tdcall.o tdx-tests.o

arch/x86/coco/tdx/tdx-tests.c

@@ -0,0 +1,155 @@
+/*
+ * Test for x86 debugging facilities
+ *
+ * Loosely based on the kvm-unit-tests/x86/debug.c by
+ *
+ * Copyright (c) Siemens AG, 2014
+ *
+ * Authors:
+ *  Jan Kiszka <jan.kiszka@siemens.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/notifier.h>
+#include <linux/printk.h>
+#include <linux/kdebug.h>
+#include <linux/types.h>
+#include <asm/debugreg.h>
+
+static volatile unsigned long db_addr[10], dr6[10];
+static volatile unsigned int n;
+
+static void report(bool cond, const char *fmt, ...)
+{
+	struct va_format vaf;
+	va_list ap;
+
+	va_start(ap, fmt);
+	vaf.fmt = fmt;
+	vaf.va = &ap;
+	pr_info("KVM-debug: %s: %pV\n", cond ? "PASS" : "FAIL", &vaf);
+	va_end(ap);
+}
+
+static int kvm_unit_test_debug_notify(struct notifier_block *nb, unsigned long val, void *data)
+{
+	struct die_args *args = data;
+	struct pt_regs *regs = args->regs;
+
+	if (val != DIE_DEBUG)
+		return NOTIFY_DONE;
+
+	db_addr[n] = regs->ip;
+
+	/* Original test likes to see DR6_RESERVED set, so oblige */
+	dr6[n] = *(unsigned long *)args->err | DR6_RESERVED;
+
+	if (dr6[n] & 0x1)
+		regs->flags |= X86_EFLAGS_RF;
+
+	if (++n >= 10) {
+		regs->flags &= ~X86_EFLAGS_TF;
+		set_debugreg(0x00000400, 7);
+	}
+
+	return NOTIFY_STOP;
+}
+
+static struct notifier_block kvm_unit_test_debug_notifier = {
+	.notifier_call	= kvm_unit_test_debug_notify,
+	.priority	= -INT_MAX,
+};
+
+int __init kvm_unit_test_debug_init(void)
+{
+	unsigned long start;
+
+	register_die_notifier(&kvm_unit_test_debug_notifier);
+
+	/*
+	 * cpuid and rdmsr (among others) trigger VM exits and are then
+	 * emulated. Test that single stepping works on emulated instructions.
+	 * 
+	 * With TDX, there are 2 possibilities: instruction is either emulated
+	 * by the TDX module or the #VE handler. In the former case, it's the
+	 * job of the TDX module to raise #DB.
+	 */
+	n = 0;
+
+	set_debugreg(0, 6);
+	/* First, test #VE emulated instructions: CPUID 0x5 and RDMSR 0x8b */
+	asm volatile(
+		"pushf\n\t"
+		"pop %%rax\n\t"
+		"and $~(1<<8),%%rax\n\t"
+		"push %%rax\n\t"
+		"or $(1<<8),%%rax\n\t"
+		"push %%rax\n\t"
+		"lea (%%rip),%0\n\t"
+		"popf\n\t"
+		"movl $0x5,%%eax\n\t"
+		"cpuid\n\t"
+		"movl $0x8b,%%ecx\n\t"
+		"rdmsr\n\t"
+		"popf\n\t"
+		: "=r" (start) : : "rax", "ebx", "ecx", "edx");
+	report(n == 5 &&
+	       db_addr[0] == start + 6 && dr6[0] == 0xffff4ff0 &&
+	       db_addr[1] == start + 6 + 2 && dr6[1] == 0xffff4ff0 &&
+	       db_addr[2] == start + 6 + 2 + 5 && dr6[2] == 0xffff4ff0 &&
+	       db_addr[3] == start + 6 + 2 + 5 + 2 && dr6[3] == 0xffff4ff0 &&
+	       db_addr[4] == start + 6 + 2 + 5 + 2 + 1 && dr6[4] == 0xffff4ff0,
+	       "single step #VE emulated instructions");
+
+	n = 0;
+	set_debugreg(0, 6);
+	/* Second, test #VE emulated CPUID 0x0 (as in the original unit test) */
+	asm volatile(
+		"pushf\n\t"
+		"pop %%rax\n\t"
+		"and $~(1<<8),%%rax\n\t"
+		"push %%rax\n\t"
+		"or $(1<<8),%%rax\n\t"
+		"push %%rax\n\t"
+		"lea (%%rip),%0\n\t"
+		"popf\n\t"
+		"xor %%rax, %%rax\n\t"
+		"cpuid\n\t"
+		"popf\n\t"
+		: "=r" (start) : : "rax", "ebx", "ecx", "edx");
+	report(n == 3 &&
+	       db_addr[0] == start + 4 && dr6[0] == 0xffff4ff0 &&
+	       db_addr[1] == start + 4 + 2 && dr6[1] == 0xffff4ff0 &&
+	       db_addr[2] == start + 4 + 2 + 1 && dr6[2] == 0xffff4ff0,
+	       "single step TDX module emulated CPUID 0");
+
+	n = 0;
+	set_debugreg(0, 6);
+	/* Third, test #VE emulated RDMSR 0x1a0 (as in the original unit test) */
+	asm volatile(
+		"pushf\n\t"
+		"pop %%rax\n\t"
+		"and $~(1<<8),%%rax\n\t"
+		"push %%rax\n\t"
+		"or $(1<<8),%%rax\n\t"
+		"push %%rax\n\t"
+		"lea (%%rip),%0\n\t"
+		"popf\n\t"
+		"movl $0x1a0,%%ecx\n\t"
+		"rdmsr\n\t"
+		"popf\n\t"
+		: "=r" (start) : : "rax", "ebx", "ecx", "edx");
+	report(n == 3 &&
+	       db_addr[0] == start + 6 && dr6[0] == 0xffff4ff0 &&
+	       db_addr[1] == start + 6 + 2 && dr6[1] == 0xffff4ff0 &&
+	       db_addr[2] == start + 6 + 2 + 1 && dr6[2] == 0xffff4ff0,
+	       "single step TDX module emulated RDMSR 0x1a0");
+
+	unregister_die_notifier(&kvm_unit_test_debug_notifier);
+
+	return 0;
+}
+core_initcall(kvm_unit_test_debug_init);