Re: [PATCH v4 1/1] x86/acpi, x86/boot: Add multiprocessor wake-up support

[Kuppuswamy, Sathyanarayanan]

On 5/21/21 7:45 AM, Peter Zijlstra wrote:
> On Thu, May 13, 2021 at 02:37:32PM -0700, Kuppuswamy Sathyanarayanan wrote:
> > +static int acpi_wakeup_cpu(int apicid, unsigned long start_ip)
> > +{
> > +	u8 timeout = 0xFF;
> > +
> > +	/* Remap mailbox memory only for the first call to acpi_wakeup_cpu() */
> > +	if (physids_empty(apic_id_wakemap))
> > +		acpi_mp_wake_mailbox = memremap(acpi_mp_wake_mailbox_paddr,
> > +						sizeof(*acpi_mp_wake_mailbox),
> > +						MEMREMAP_WB);
>
> { } for being multi-line

Yes. I will fix it.

> > +	/*
> > +	 * According to the ACPI specification r6.4, sec 5.2.12.19, the
> > +	 * mailbox-based wakeup mechanism cannot be used more than once
> > +	 * for the same CPU, so skip sending wake commands to already
> > +	 * awake CPU.
> > +	 */
> > +	if (physid_isset(apicid, apic_id_wakemap)) {
> > +		pr_err("CPU already awake (APIC ID %x), skipping wakeup\n",
> > +		       apicid);
> > +		return -EINVAL;
> > +	}
> > +
> > +
> > +	/*
> > +	 * Mailbox memory is shared between firmware and OS. Firmware will
> > +	 * listen on mailbox command address, and once it receives the wakeup
> > +	 * command, CPU associated with the given apicid will be booted. So,
> > +	 * the value of apic_id and wakeup_vector has to be set before updating
> > +	 * the wakeup command. So use WRITE_ONCE to let the compiler know about
> > +	 * it and preserve the order of writes.
> > +	 */
> > +	WRITE_ONCE(acpi_mp_wake_mailbox->apic_id, apicid);
> > +	WRITE_ONCE(acpi_mp_wake_mailbox->wakeup_vector, start_ip);
> > +	WRITE_ONCE(acpi_mp_wake_mailbox->command, ACPI_MP_WAKE_COMMAND_WAKEUP);
>
> Do those want to be smp_store_release(), in addition to being a volatile
> write, those also include compiler barriers to make sure the compiler
> doesn't lift stuff around.

I think we can use smp_store_release(). Let me test and add it in next
version.

> > +
> > +	/*
> > +	 * After writing wakeup command, wait for maximum timeout of 0xFF
> > +	 * for firmware to reset the command address back zero to indicate
> > +	 * the successful reception of command.
> > +	 * NOTE: 255 as timeout value is decided based on our experiments.
> > +	 *
> > +	 * XXX: Change the timeout once ACPI specification comes up with
> > +	 *      standard maximum timeout value.
> > +	 */
> > +	while (READ_ONCE(acpi_mp_wake_mailbox->command) && timeout--)
> > +		cpu_relax();
>
> What's the unit of the timeout? The mailbox reads, the PAUSE
> instruction?

Read mailbox memory, timeout dec and then pause. Its more like busy wait loop.

And timeout count is decided based on our experiments. Once spec defines a
standard, we can modify it.

> > +
> > +	if (timeout) {
> > +		/*
> > +		 * If the CPU wakeup process is successful, store the
> > +		 * status in apic_id_wakemap to prevent re-wakeup
> > +		 * requests.
> > +		 */
> > +		physid_set(apicid, apic_id_wakemap);
> > +		return 0;
> > +	}
> > +
> > +	/* If timed out (timeout == 0), return error */
> > +	return -EIO;
> > +}
> > +
> >   #endif				/*CONFIG_X86_LOCAL_APIC */

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer