[PATCH kvm-unit-tests] x86: Test task-switch with cs.rpl != cs.dpl

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Commit 5045b46803 added a check that cs.dpl equals cs.rpl during task-switch.
This is a wrong check, and this test introduces a test in which cs.dpl !=
cs.rpl. To do so, it configures tss.cs to be conforming with rpl=3 and dpl=0.
Since the cpl after calling is 3, it does not make any prints in the callee.

Signed-off-by: Nadav Amit <namit@xxxxxxxxxxxxxxxxx>
---
 x86/taskswitch2.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/x86/taskswitch2.c b/x86/taskswitch2.c
index 92fc941..d96853f 100644
--- a/x86/taskswitch2.c
+++ b/x86/taskswitch2.c
@@ -7,6 +7,8 @@
 
 #define MAIN_TSS_SEL (FIRST_SPARE_SEL + 0)
 #define VM86_TSS_SEL (FIRST_SPARE_SEL + 8)
+#define USER_CS_SEL  (FIRST_SPARE_SEL + 16)
+#define USER_DS_SEL  (FIRST_SPARE_SEL + 24)
 
 static volatile int test_count;
 static volatile unsigned int test_divider;
@@ -102,6 +104,14 @@ start:
 	goto start;
 }
 
+static void user_tss(void)
+{
+start:
+	test_count++;
+	asm volatile ("iret");
+	goto start;
+}
+
 void test_kernel_mode_int()
 {
 	unsigned int res;
@@ -201,6 +211,18 @@ void test_kernel_mode_int()
 	asm volatile ("ljmp $" xstr(TSS_INTR) ", $0xf4f4f4f4");
 	printf("Jump back succeeded\n");
 	report("ljmp", test_count == 1);
+
+	/* test lcall with conforming segment, cs.dpl != cs.rpl */
+	test_count = 0;
+	set_intr_task_gate(0, user_tss);
+
+	tss_intr.cs = USER_CS_SEL | 3;
+	tss_intr.ss = USER_DS_SEL | 3;
+	tss_intr.ds = tss_intr.gs = tss_intr.fs = tss_intr.ss;
+	set_gdt_entry(USER_CS_SEL, 0, 0xffffffff, 0x9f, 0xc0);
+	set_gdt_entry(USER_DS_SEL, 0, 0xffffffff, 0xf3, 0xc0);
+	asm volatile("lcall $" xstr(TSS_INTR) ", $0xf4f4f4f4");
+	report("lcall when cs.rpl != cs.dpl", test_count == 1);
 }
 
 void test_vm86_switch(void)
-- 
1.9.1

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