Subject: Re: [kvm-unit-tests PATCH v9 21/31] powerpc: Add timebase tests

From: "Nicholas Piggin" <[email-masked]>
Date: Wed, 05 Jun 2024 11:04:09 +1000

On Tue Jun 4, 2024 at 4:12 PM AEST, Thomas Huth wrote:
> On 04/05/2024 14.28, Nicholas Piggin wrote:
> > This has a known failure on QEMU TCG machines where the decrementer
> > interrupt is not lowered when the DEC wraps from -ve to +ve.
>
> Would it then make sense to mark the test with accel = kvm to avoid the test 
> failure when running with TCG?

Still want to test it with TCG though, which had quite a few bugs I
used these tests to fix. It is marked as known fail for TCG (once the
later host accel patch is merged).

> > +/* Check amount of CPUs nodes that have the TM flag */
> > +static int find_dec_bits(void)
> > +{
> > +	int ret;
> > +
> > +	ret = dt_for_each_cpu_node(cpu_dec_bits, NULL);
>
> What sense does it make to run this for each CPU node if the cpu_dec_bits 
> function always overwrites the global dec_bits variable?
> Wouldn't it be sufficient to run this for the first node only? Or should the 
> cpu_dec_bits function maybe check that all nodes have the same value?

Yeah it could use first subnode.

> > +	if (ret < 0)
> > +		return ret;
> > +
> > +	return dec_bits;
> > +}
> > +
> > +
> > +static bool do_migrate = false;
> > +static volatile bool got_interrupt;
> > +static volatile struct pt_regs recorded_regs;
> > +
> > +static uint64_t dec_max;
> > +static uint64_t dec_min;
> > +
> > +static void test_tb(int argc, char **argv)
> > +{
> > +	uint64_t tb;
> > +
> > +	tb = get_tb();
> > +	if (do_migrate)
> > +		migrate();
> > +	report(get_tb() >= tb, "timebase is incrementing");
>
> If you use >= for testing, it could also mean that the TB stays at the same 
> value, so "timebase is incrementing" sounds misleading. Maybe rather 
> "timebase is not decreasing" ? Or wait a little bit, then check with ">" only ?

Yeah, maybe first immediate test could ensure it doesn't go
backward, then wait a bit and check it increments.

> > +}
> > +
> > +static void dec_stop_handler(struct pt_regs *regs, void *data)
> > +{
> > +	mtspr(SPR_DEC, dec_max);
> > +}
> > +
> > +static void dec_handler(struct pt_regs *regs, void *data)
> > +{
> > +	got_interrupt = true;
> > +	memcpy((void *)&recorded_regs, regs, sizeof(struct pt_regs));
> > +	regs->msr &= ~MSR_EE;
> > +}
> > +
> > +static void test_dec(int argc, char **argv)
> > +{
> > +	uint64_t tb1, tb2, dec;
> > +	int i;
> > +
> > +	handle_exception(0x900, &dec_handler, NULL);
> > +
> > +	for (i = 0; i < 100; i++) {
> > +		tb1 = get_tb();
> > +		mtspr(SPR_DEC, dec_max);
> > +		dec = mfspr(SPR_DEC);
> > +		tb2 = get_tb();
> > +		if (tb2 - tb1 < dec_max - dec)
> > +			break;
> > +	}
> > +	/* POWER CPUs can have a slight (few ticks) variation here */
> > +	report_kfail(true, tb2 - tb1 >= dec_max - dec, "decrementer remains within TB after mtDEC");
> > +
> > +	tb1 = get_tb();
> > +	mtspr(SPR_DEC, dec_max);
> > +	mdelay(1000);
> > +	dec = mfspr(SPR_DEC);
> > +	tb2 = get_tb();
> > +	report(tb2 - tb1 >= dec_max - dec, "decrementer remains within TB after 1s");
> > +
> > +	mtspr(SPR_DEC, dec_max);
> > +	local_irq_enable();
> > +	local_irq_disable();
> > +	if (mfspr(SPR_DEC) <= dec_max) {
> > +		report(!got_interrupt, "no interrupt on decrementer positive");
> > +	}
> > +	got_interrupt = false;
> > +
> > +	mtspr(SPR_DEC, 1);
> > +	mdelay(100); /* Give the timer a chance to run */
> > +	if (do_migrate)
> > +		migrate();
> > +	local_irq_enable();
> > +	local_irq_disable();
> > +	report(got_interrupt, "interrupt on decrementer underflow");
> > +	got_interrupt = false;
> > +
> > +	if (do_migrate)
> > +		migrate();
> > +	local_irq_enable();
> > +	local_irq_disable();
> > +	report(got_interrupt, "interrupt on decrementer still underflown");
> > +	got_interrupt = false;
> > +
> > +	mtspr(SPR_DEC, 0);
> > +	mdelay(100); /* Give the timer a chance to run */
> > +	if (do_migrate)
> > +		migrate();
> > +	local_irq_enable();
> > +	local_irq_disable();
> > +	report(got_interrupt, "DEC deal with set to 0");
> > +	got_interrupt = false;
> > +
> > +	/* Test for level-triggered decrementer */
> > +	mtspr(SPR_DEC, -1ULL);
> > +	if (do_migrate)
> > +		migrate();
> > +	local_irq_enable();
> > +	local_irq_disable();
> > +	report(got_interrupt, "interrupt on decrementer write MSB");
> > +	got_interrupt = false;
> > +
> > +	mtspr(SPR_DEC, dec_max);
> > +	local_irq_enable();
> > +	if (do_migrate)
> > +		migrate();
> > +	mtspr(SPR_DEC, -1);
> > +	local_irq_disable();
> > +	report(got_interrupt, "interrupt on decrementer write MSB with irqs on");
> > +	got_interrupt = false;
> > +
> > +	mtspr(SPR_DEC, dec_min + 1);
> > +	mdelay(100);
> > +	local_irq_enable();
> > +	local_irq_disable();
> > +	/* TCG does not model this correctly */
> > +	report_kfail(true, !got_interrupt, "no interrupt after wrap to positive");
> > +	got_interrupt = false;
> > +
> > +	handle_exception(0x900, NULL, NULL);
> > +}
> > +
> > +static void test_hdec(int argc, char **argv)
> > +{
> > +	uint64_t tb1, tb2, hdec;
> > +
> > +	if (!machine_is_powernv()) {
> > +		report_skip("skipping on !powernv machine");
>
> I'd rather say "not running on powernv machine"

Okay.

Thanks,
Nick