linux-kernel - Re: [PATCH] Use x2apic_supported() in the default_apic_id

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Message-ID: <4F626E6C.5010809@numascale.com>
Date:	Thu, 15 Mar 2012 23:34:20 +0100
From:	Steffen Persvold <sp@...ascale.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Daniel J Blueman <daniel@...ascale-asia.com>,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] Use x2apic_supported() in the default_apic_id_valid()
 function.

On 3/15/12 22:21 , Suresh Siddha wrote:
> On Thu, 2012-03-15 at 13:23 -0700, Yinghai Lu wrote:
>> On Thu, Mar 15, 2012 at 11:03 AM, Steffen Persvold<sp@...ascale.com>  wrote:
>>> Use x2apic_supported() in the default_apic_id_valid() function. If x2apic mode is disabled (via nox2apic for example), x2apic_supported() will return false.
>>>
>>> This allows us to substitute the check in arch/x86/kernel/acpi/boot.c::acpi_parse_x2apic and avoid feigning the x2apic cpu feature in the NumaChip apic code.
>>>
>>> Signed-off-by: Steffen Persvold<sp@...ascale.com>
>>> Reviewed-by: Daniel J Blueman<daniel@...ascale-asia.com>
>>
>> I double checked on system with x2apic preenabled,
>> nox2apic in boot command line still works well and it
>> skips starting APs with apic id>  255.
>>
>> Acked-by: Yinghai Lu<yinghai@...nel.org>
>

Suresh,

> This breaks the smpboot check if enabling interrupt-remapping/x2apic
> fails on a platform. We will be in xapic mode and we don't clear the
> x2apic cpufeature bit in this case and as such smpboot check will fail.
>
> So this change breaks the commit
> c284b42abadbb22083bfde24d308899c08d44ffa.
>

I was afraid of that.

> I think the right thing is to have two different apid_id_valid checks
> one for xapic driver (apic_flat_64.c) and another for x2apic driver
> (x2apic_phys/cluster.c) and that way, x2apic MADT entries will be parsed
> only if bios has handed over the OS in x2apic mode or if we have
> selected the numachip model.
>

Is my understanding of your suggestion correct that in 
x2apic_phys/cluster.c we add the following apic_id_valid() function :

static int x2apic_apic_id_valid(int apicid)
{
  	return x2apic_mode || (apicid < 255);
}

?

Considering that this function (apic->apic_id_valid()) is called already 
in the acpi/boot.c::acpi_parse_x2apic() function is it sufficient enough 
to test for x2apic_mode ? Yinghai indicated that x2apic_mode was not set 
at this point, thus it was testing cpu_has_x2apic instead ?

I must admit that I am not familiar enough with the APIC/ACPI code base 
to determine the sequence of events here (i.e MADT parsing, enabling of 
x2apic mode etc. etc.).

Please advice.

Kind regards,
Steffen
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