[kvm-unit-tests v2 2/8] x86: emulator: disable test_lldt

[Peter Feiner <pfeiner@google.com>]

When compiled with -fno-omit-frame-pointer, test_lldt causes a triple
fault, which insta-reboots the guest. I couldn't figure out why the
triple-fault was happening, so I disabled the test outright. Knowing
that the other emulator.c tests pass is more useful than a VM stuck in
a reboot loop.

On x86-64, this test was adding little value since it was broken to
begin with: an LDT descriptor is 16 bytes on x86-64. The GDT limit
was also bogus. I've fixed these problems for posterity.

N.B. I only tested this on Linux 4.3.5. Perhaps lldt emulation has
been fixed since then.

Signed-off-by: Peter Feiner <pfeiner@google.com>
---
 x86/emulator.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/x86/emulator.c b/x86/emulator.c
index d990961..3730721 100644
--- a/x86/emulator.c
+++ b/x86/emulator.c
@@ -1006,10 +1006,17 @@ static void test_sreg(volatile uint16_t *mem)
     write_ss(ss);
 }
 
+/* Broken emulation causes triple fault, which skips the other tests. */
+#if 0
 static void test_lldt(volatile uint16_t *mem)
 {
-    u64 gdt[] = { 0, 0x0000f82000000ffffull /* ldt descriptor */ };
-    struct descriptor_table_ptr gdt_ptr = { .limit = 0xffff, .base = (ulong)&gdt };
+    u64 gdt[] = { 0, /* null descriptor */
+#ifdef __X86_64__
+		  0, /* ldt descriptor is 16 bytes in long mode */
+#endif
+		  0x0000f82000000ffffull /* ldt descriptor */ };
+    struct descriptor_table_ptr gdt_ptr = { .limit = sizeof(gdt) - 1,
+					    .base = (ulong)&gdt };
     struct descriptor_table_ptr orig_gdt;
 
     cli();
@@ -1021,6 +1028,7 @@ static void test_lldt(volatile uint16_t *mem)
     sti();
     report("lldt", sldt() == *mem);
 }
+#endif
 
 static void test_ltr(volatile uint16_t *mem)
 {
@@ -1139,7 +1147,7 @@ int main()
 	test_shld_shrd(mem);
 	//test_lgdt_lidt(mem);
 	test_sreg(mem);
-	test_lldt(mem);
+	//test_lldt(mem);
 	test_ltr(mem);
 	test_cmov(mem);
 
-- 
2.7.0.rc3.207.g0ac5344