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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.02.18 at 06:49, <chao.gao@xxxxxxxxx> wrote:
> 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.

A flag? Where? Why isn't 256+ vCPU-s on its own sufficient to tell
that the mode needs to be switched?

Jan

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