clockevents/drivers/i8253: Fix stop sequence for timer 0

clockevents/drivers/i8253: Fix stop sequence for timer 0

According to the data sheet, writing the MODE register should stop the
counter (and thus the interrupts). This appears to work on real hardware,
at least modern Intel and AMD systems. It should also work on Hyper-V.

However, on some buggy virtual machines the mode change doesn't have any
effect until the counter is subsequently loaded (or perhaps when the IRQ
next fires).

So, set MODE 0 and then load the counter, to ensure that those buggy VMs
do the right thing and the interrupts stop. And then write MODE 0 *again*
to stop the counter on compliant implementations too.

Apparently, Hyper-V keeps firing the IRQ *repeatedly* even in mode zero
when it should only happen once, but the second MODE write stops that too.

Userspace test program (mostly written by tglx):
=====
 #include <stdio.h>
 #include <unistd.h>
 #include <stdlib.h>
 #include <stdint.h>
 #include <sys/io.h>

static __always_inline void __out##bwl(type value, uint16_t port)	\
{									\
	asm volatile("out" #bwl " %" #bw "0, %w1"			\
		     : : "a"(value), "Nd"(port));			\
}									\
									\
static __always_inline type __in##bwl(uint16_t port)			\
{									\
	type value;							\
	asm volatile("in" #bwl " %w1, %" #bw "0"			\
		     : "=a"(value) : "Nd"(port));			\
	return value;							\
}

BUILDIO(b, b, uint8_t)

 #define inb __inb
 #define outb __outb

 #define PIT_MODE	0x43
 #define PIT_CH0	0x40
 #define PIT_CH2	0x42

static int is8254;

static void dump_pit(void)
{
	if (is8254) {
		// Latch and output counter and status
		outb(0xC2, PIT_MODE);
		printf("%02x %02x %02x\n", inb(PIT_CH0), inb(PIT_CH0), inb(PIT_CH0));
	} else {
		// Latch and output counter
		outb(0x0, PIT_MODE);
		printf("%02x %02x\n", inb(PIT_CH0), inb(PIT_CH0));
	}
}

int main(int argc, char* argv[])
{
	int nr_counts = 2;

	if (argc > 1)
		nr_counts = atoi(argv[1]);

	if (argc > 2)
		is8254 = 1;

	if (ioperm(0x40, 4, 1) != 0)
		return 1;

	dump_pit();

	printf("Set oneshot\n");
	outb(0x38, PIT_MODE);
	outb(0x00, PIT_CH0);
	outb(0x0F, PIT_CH0);

	dump_pit();
	usleep(1000);
	dump_pit();

	printf("Set periodic\n");
	outb(0x34, PIT_MODE);
	outb(0x00, PIT_CH0);
	outb(0x0F, PIT_CH0);

	dump_pit();
	usleep(1000);
	dump_pit();
	dump_pit();
	usleep(100000);
	dump_pit();
	usleep(100000);
	dump_pit();

	printf("Set stop (%d counter writes)\n", nr_counts);
	outb(0x30, PIT_MODE);
	while (nr_counts--)
		outb(0xFF, PIT_CH0);

	dump_pit();
	usleep(100000);
	dump_pit();
	usleep(100000);
	dump_pit();

	printf("Set MODE 0\n");
	outb(0x30, PIT_MODE);

	dump_pit();
	usleep(100000);
	dump_pit();
	usleep(100000);
	dump_pit();

	return 0;
}
=====

Suggested-by: Sean Christopherson <seanjc@google.com>
Co-developed-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mhkelley@outlook.com>
Link: https://lore.kernel.org/all/20240802135555.564941-2-dwmw2@infradead.org