KVM: selftests: do not blindly clobber registers in guest asm

The guest_code of sync_regs_test is assuming that the compiler will not
touch %r11 outside the asm that increments it, which is a bit brittle.
Instead, we can increment a variable and use a dummy asm to ensure the
increment is not optimized away.  However, we also need to use a
callee-save register or the compiler will insert a save/restore around
the vmexit, breaking the whole idea behind the test.

(Yes, "if it ain't broken...", but I would like the test to be clean
before it is copied into the upcoming s390 selftests).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/tools/testing/selftests/kvm/x86_64/sync_regs_test.c b/tools/testing/selftests/kvm/x86_64/sync_regs_test.c
index c8478ce..25cacd3 100644 (file)
--- a/tools/testing/selftests/kvm/x86_64/sync_regs_test.c
+++ b/tools/testing/selftests/kvm/x86_64/sync_regs_test.c
@@ -25,9 +25,15 @@
 
 void guest_code(void)
 {
+       /*
+        * use a callee-save register, otherwise the compiler
+        * saves it around the call to GUEST_SYNC.
+        */
+       register u32 stage asm("rbx");
        for (;;) {
                GUEST_SYNC(0);
-               asm volatile ("inc %r11");
+               stage++;
+               asm volatile ("" : : "r" (stage));
        }
 }
 
@@ -147,7 +153,7 @@ int main(int argc, char *argv[])
        compare_vcpu_events(&events, &run->s.regs.events);
 
        /* Set and verify various register values. */
-       run->s.regs.regs.r11 = 0xBAD1DEA;
+       run->s.regs.regs.rbx = 0xBAD1DEA;
        run->s.regs.sregs.apic_base = 1 << 11;
        /* TODO run->s.regs.events.XYZ = ABC; */
 
@@ -158,9 +164,9 @@ int main(int argc, char *argv[])
                    "Unexpected exit reason: %u (%s),\n",
                    run->exit_reason,
                    exit_reason_str(run->exit_reason));
-       TEST_ASSERT(run->s.regs.regs.r11 == 0xBAD1DEA + 1,
-                   "r11 sync regs value incorrect 0x%llx.",
-                   run->s.regs.regs.r11);
+       TEST_ASSERT(run->s.regs.regs.rbx == 0xBAD1DEA + 1,
+                   "rbx sync regs value incorrect 0x%llx.",
+                   run->s.regs.regs.rbx);
        TEST_ASSERT(run->s.regs.sregs.apic_base == 1 << 11,
                    "apic_base sync regs value incorrect 0x%llx.",
                    run->s.regs.sregs.apic_base);
@@ -179,15 +185,15 @@ int main(int argc, char *argv[])
         */
        run->kvm_valid_regs = TEST_SYNC_FIELDS;
        run->kvm_dirty_regs = 0;
-       run->s.regs.regs.r11 = 0xDEADBEEF;
+       run->s.regs.regs.rbx = 0xDEADBEEF;
        rv = _vcpu_run(vm, VCPU_ID);
        TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
                    "Unexpected exit reason: %u (%s),\n",
                    run->exit_reason,
                    exit_reason_str(run->exit_reason));
-       TEST_ASSERT(run->s.regs.regs.r11 != 0xDEADBEEF,
-                   "r11 sync regs value incorrect 0x%llx.",
-                   run->s.regs.regs.r11);
+       TEST_ASSERT(run->s.regs.regs.rbx != 0xDEADBEEF,
+                   "rbx sync regs value incorrect 0x%llx.",
+                   run->s.regs.regs.rbx);
 
        /* Clear kvm_valid_regs bits and kvm_dirty_bits.
         * Verify s.regs values are not overwritten with existing guest values
@@ -195,21 +201,21 @@ int main(int argc, char *argv[])
         */
        run->kvm_valid_regs = 0;
        run->kvm_dirty_regs = 0;
-       run->s.regs.regs.r11 = 0xAAAA;
-       regs.r11 = 0xBAC0;
+       run->s.regs.regs.rbx = 0xAAAA;
+       regs.rbx = 0xBAC0;
        vcpu_regs_set(vm, VCPU_ID, &regs);
        rv = _vcpu_run(vm, VCPU_ID);
        TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
                    "Unexpected exit reason: %u (%s),\n",
                    run->exit_reason,
                    exit_reason_str(run->exit_reason));
-       TEST_ASSERT(run->s.regs.regs.r11 == 0xAAAA,
-                   "r11 sync regs value incorrect 0x%llx.",
-                   run->s.regs.regs.r11);
+       TEST_ASSERT(run->s.regs.regs.rbx == 0xAAAA,
+                   "rbx sync regs value incorrect 0x%llx.",
+                   run->s.regs.regs.rbx);
        vcpu_regs_get(vm, VCPU_ID, &regs);
-       TEST_ASSERT(regs.r11 == 0xBAC0 + 1,
-                   "r11 guest value incorrect 0x%llx.",
-                   regs.r11);
+       TEST_ASSERT(regs.rbx == 0xBAC0 + 1,
+                   "rbx guest value incorrect 0x%llx.",
+                   regs.rbx);
 
        /* Clear kvm_valid_regs bits. Verify s.regs values are not overwritten
         * with existing guest values but that guest values are overwritten
@@ -217,19 +223,19 @@ int main(int argc, char *argv[])
         */
        run->kvm_valid_regs = 0;
        run->kvm_dirty_regs = TEST_SYNC_FIELDS;
-       run->s.regs.regs.r11 = 0xBBBB;
+       run->s.regs.regs.rbx = 0xBBBB;
        rv = _vcpu_run(vm, VCPU_ID);
        TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
                    "Unexpected exit reason: %u (%s),\n",
                    run->exit_reason,
                    exit_reason_str(run->exit_reason));
-       TEST_ASSERT(run->s.regs.regs.r11 == 0xBBBB,
-                   "r11 sync regs value incorrect 0x%llx.",
-                   run->s.regs.regs.r11);
+       TEST_ASSERT(run->s.regs.regs.rbx == 0xBBBB,
+                   "rbx sync regs value incorrect 0x%llx.",
+                   run->s.regs.regs.rbx);
        vcpu_regs_get(vm, VCPU_ID, &regs);
-       TEST_ASSERT(regs.r11 == 0xBBBB + 1,
-                   "r11 guest value incorrect 0x%llx.",
-                   regs.r11);
+       TEST_ASSERT(regs.rbx == 0xBBBB + 1,
+                   "rbx guest value incorrect 0x%llx.",
+                   regs.rbx);
 
        kvm_vm_free(vm);