Re: [Xen-devel] [RFC Patch v4 4/8] hvmloader: boot cpu through broadcast

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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?

Thanks, Roger.

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