On Fri, Feb 23, 2018 at 04:42:10PM +0000, Roger Pau Monné wrote:
>On Wed, Dec 06, 2017 at 03:50:10PM +0800, Chao Gao wrote:
>> Intel SDM Extended XAPIC (X2APIC) -> "Initialization by System Software"
>> has the following description:
>> 
>> "The ACPI interfaces for the x2APIC are described in Section 5.2, “ACPI 
>> System
>> Description Tables,” of the Advanced Configuration and Power Interface
>> Specification, Revision 4.0a (http://www.acpi.info/spec.htm). The default
>> behavior for BIOS is to pass the control to the operating system with the
>> local x2APICs in xAPIC mode if all APIC IDs reported by CPUID.0BH:EDX are 
>> less
>> than 255, and in x2APIC mode if there are any logical processor reporting an
>> APIC ID of 255 or greater."
>> 
>> In this patch, hvmloader enables x2apic mode for all vcpus if there are cpus
>> with APIC ID > 255. To wake up processors whose APIC ID is greater than 255,
>> the SIPI is broadcasted to all APs. It is the way how Seabios wakes up APs.
>> APs may compete for the stack, thus a lock is introduced to protect the 
>> stack.
>
>Hm, how are we going to deal with this on PVH? hvmloader doesn't run
>for PVH guests, hence it seems like switching to x2APIC mode should be
>done somewhere else that shared between HVM and PVH.
>
>Maybe the hypercall that sets the number of vCPUs should change the
>APIC mode?

Yes. A flag can be passed when setting the maximum number of vCPUs. Xen
will switch all local APICs to x2APIC mode or xAPIC mode according to
the flag.

Thanks
Chao

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