selftests: kvm: Check that TSC page value is small after KVM_SET_CLOCK(0)

Add a test for the issue when KVM_SET_CLOCK(0) call could cause
TSC page value to go very big because of a signedness issue around
hv_clock->system_time.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210326155551.17446-3-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/x86_64/hyperv_clock.c

@@ -80,19 +80,24 @@ static inline void check_tsc_msr_rdtsc(void)
 	GUEST_ASSERT(delta_ns * 100 < (t2 - t1) * 100);
 }
 
+static inline u64 get_tscpage_ts(struct ms_hyperv_tsc_page *tsc_page)
+{
+	return mul_u64_u64_shr64(rdtsc(), tsc_page->tsc_scale) + tsc_page->tsc_offset;
+}
+
 static inline void check_tsc_msr_tsc_page(struct ms_hyperv_tsc_page *tsc_page)
 {
 	u64 r1, r2, t1, t2;
 
 	/* Compare TSC page clocksource with HV_X64_MSR_TIME_REF_COUNT */
-	t1 = mul_u64_u64_shr64(rdtsc(), tsc_page->tsc_scale) + tsc_page->tsc_offset;
+	t1 = get_tscpage_ts(tsc_page);
 	r1 = rdmsr(HV_X64_MSR_TIME_REF_COUNT);
 
 	/* 10 ms tolerance */
 	GUEST_ASSERT(r1 >= t1 && r1 - t1 < 100000);
 	nop_loop();
 
-	t2 = mul_u64_u64_shr64(rdtsc(), tsc_page->tsc_scale) + tsc_page->tsc_offset;
+	t2 = get_tscpage_ts(tsc_page);
 	r2 = rdmsr(HV_X64_MSR_TIME_REF_COUNT);
 	GUEST_ASSERT(r2 >= t1 && r2 - t2 < 100000);
 }
@@ -130,7 +135,11 @@ static void guest_main(struct ms_hyperv_tsc_page *tsc_page, vm_paddr_t tsc_page_
 
 	tsc_offset = tsc_page->tsc_offset;
 	/* Call KVM_SET_CLOCK from userspace, check that TSC page was updated */
+
 	GUEST_SYNC(7);
+	/* Sanity check TSC page timestamp, it should be close to 0 */
+	GUEST_ASSERT(get_tscpage_ts(tsc_page) < 100000);
+
 	GUEST_ASSERT(tsc_page->tsc_offset != tsc_offset);
 
 	nop_loop();