Re: [Xen-devel] Workings/effectiveness of the xen-acpi-processor driver

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

On 03.05.2012 00:09, Boris Ostrovsky wrote:
> On 05/02/2012 05:41 PM, Konrad Rzeszutek Wilk wrote:
>> On Wed, May 02, 2012 at 02:31:07PM -0700, Boris Ostrovsky wrote:
>>> On 05/02/2012 01:14 PM, Konrad Rzeszutek Wilk wrote:
>>>> On Wed, May 02, 2012 at 01:06:34PM -0400, Boris Ostrovsky wrote:
>>>>> On 05/02/2012 12:08 PM, Konrad Rzeszutek Wilk wrote:
>>>>>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
>>>>>> index a8f8844..d816448 100644
>>>>>> --- a/arch/x86/xen/enlighten.c
>>>>>> +++ b/arch/x86/xen/enlighten.c
>>>>>> @@ -811,7 +811,29 @@ static void xen_io_delay(void)
>>>>>>   #ifdef CONFIG_X86_LOCAL_APIC
>>>>>>   static u32 xen_apic_read(u32 reg)
>>>>>>   {
>>>>>> -    return 0;
>>>>>> +    struct xen_platform_op op = {
>>>>>> +        .cmd = XENPF_get_cpuinfo,
>>>>>> +        .interface_version = XENPF_INTERFACE_VERSION,
>>>>>> +        .u.pcpu_info.xen_cpuid = 0,
>>>>>
>>>>>
>>>>> Is this always zero? This will probably solve the current problem
>>>>
>>>> Its a CPU number (not tied in to APIC or ACPI IDs).
>>>
>>> Why not use CPU number instead of zero here?
>>
>> The issue was only with the bootup CPU - so was using the Xen's
>> bootup CPU number - which is zero (as is Linux's).
> 
> I agree that for this particular problem this may be sufficient.
> 
> My concern is that in the future someone may decide to use apic_read(APIC_ID) 
> or
> read_apic_id() for some other purpose and they won't get expected result (i.e.
> on all CPUs they will get the same answer).
> 
>>
>>>
>>>>
>>>>> but I am wondering whether in the future we might hit another bug
>>>>> because this routine will return the same APICID for all VCPUs.
>>>>
>>>>   Later on it does a check for 'smp_processor_id()' - and if
>>>> that is anything but zero it will bail out.
>>>
>>> Can you point me to the check you are referring to?
>>
>> if (!xen_initial_domain() || smp_processor_id())
> 
> I don't see this line --- neither in the mainline nor in your kernel. Which
> kernel and which routine is this in?
> 
> BTW, this patch doesn't quite work, xen-acpi-processor driver fails to load 
> with
> the same error as before. I'll look at this tomorrow more carefully.
> 
> 
> -boris
> 
>>
>>
>>>
>>> -boris
>>>
>>>
>>>>
>>>> So this shoudl solve the problem for the bootup processor.
>>>>>
>>>>> -boris
>>>>>
>>>>>
>>>>>> +    };
>>>>>> +    int ret = 0;
>>>>>> +
>>>>>> +    /* Shouldn't need this as APIC is turned off for PV, and we only
>>>>>> +     * get called on the bootup processor. But just in case. */
>>>>>> +    if (!xen_initial_domain() || smp_processor_id())
>>>>>> +        return 0;
>>>>>> +
>>>>>> +    if (reg == APIC_LVR)
>>>>>> +        return 0x10;
>>>>>> +
>>>>>> +    if (reg != APIC_ID)
>>>>>> +        return 0;
>>>>>> +
>>>>>> +    ret = HYPERVISOR_dom0_op(&op);
>>>>>> +    if (ret)
>>>>>> +        return 0;
>>>>>> +
>>>>>> +    return op.u.pcpu_info.apic_id;
>>>>>>   }
>>>>>>
>>>>>>   static void xen_apic_write(u32 reg, u32 val)

I added debugging to all exit paths that could return 0 (which is what the
boot_cpu_physical_apicid is set to with that patch. Which would only leave the
case of the HV call returning the wrong value somehow...

>>>>>>
>>>>>
>>>>
>>>
>>>
>>>
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>>
> 
> 
> 
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