x86/svm: Generalize and improve selective CR0 write intercept test

In preparation for adding more test cases, make the test easier to
extend. Create a generic helper that sets an arbitrary bit in CR0,
optionally using FEP. The helper also stores the value to be written in
test->scratch, making it possible to double check if the write was
actually executed or not.

Use report_svm_guest() instead of report_fail() + exit().

Make test_sel_cr0_write_intercept() use the generic helper, and add
another test case that sets FEP to exercise the interception path in the
emulator.

Finally, in check_sel_cr0_intercept() also check that the write was not
executed by comparing CR0 value in the VMCB12 with the
value-to-be-written stored in test->scratch.

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251110232642.633672-11-yosry.ahmed@linux.dev
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: yosrym93

x86/svm_tests.c

@@ -133,27 +133,36 @@ static void prepare_sel_cr0_intercept(struct svm_test *test)
 	vmcb->control.intercept |= (1ULL << INTERCEPT_SELECTIVE_CR0);
 }
 
-static void test_sel_cr0_write_intercept(struct svm_test *test)
+static void __test_cr0_write_bit(struct svm_test *test, unsigned long bit,
+				 bool intercept, bool fep)
 {
 	unsigned long cr0;
 
-	/* read cr0, set CD, and write back */
-	cr0  = read_cr0();
-	cr0 |= X86_CR0_CD;
-	write_cr0(cr0);
+	cr0 = read_cr0();
+	cr0 |= bit;
+	test->scratch = cr0;
 
-	/*
-	 * If we are here the test failed, not sure what to do now because we
-	 * are not in guest-mode anymore so we can't trigger an intercept.
-	 * Trigger a tripple-fault for now.
-	 */
-	report_fail("sel_cr0 test. Can not recover from this - exiting");
-	exit(report_summary());
+	asm_conditional_fep_safe(fep, "mov %0,%%cr0", "r"(cr0));
+
+	/* This code should be unreachable when an intercept is expected */
+	report_svm_guest(!intercept, test, "Expected intercept on CR0 write");
+}
+
+/* MOV-to-CR0 updating CR0.CD is intercepted by the selective intercept */
+static void test_sel_cr0_write_intercept(struct svm_test *test)
+{
+	__test_cr0_write_bit(test, X86_CR0_CD, true, false);
+}
+
+static void test_sel_cr0_write_intercept_emul(struct svm_test *test)
+{
+	__test_cr0_write_bit(test, X86_CR0_CD, true, true);
 }
 
 static bool check_sel_cr0_intercept(struct svm_test *test)
 {
-	return vmcb->control.exit_code == SVM_EXIT_CR0_SEL_WRITE;
+	return vmcb->control.exit_code == SVM_EXIT_CR0_SEL_WRITE &&
+		vmcb->save.cr0 != test->scratch;
 }
 
 static void prepare_cr3_intercept(struct svm_test *test)
@@ -3461,6 +3470,9 @@ struct svm_test svm_tests[] = {
 	{ "sel cr0 write intercept", default_supported,
 	  prepare_sel_cr0_intercept, default_prepare_gif_clear,
 	  test_sel_cr0_write_intercept, default_finished, check_sel_cr0_intercept},
+	{ "sel cr0 write intercept emulate", fep_supported,
+	  prepare_sel_cr0_intercept, default_prepare_gif_clear,
+	  test_sel_cr0_write_intercept_emul, default_finished, check_sel_cr0_intercept},
 	{ "cr3 read intercept", default_supported,
 	  prepare_cr3_intercept, default_prepare_gif_clear,
 	  test_cr3_intercept, default_finished, check_cr3_intercept },