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

On Mon, Feb 26, 2018 at 01:28:07AM -0700, Jan Beulich wrote:
>>>> 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 257+ vCPU-s on its own sufficient to tell
>that the mode needs to be switched?

In struct xen_domctl_max_vcpus, a flag, like SWITCH_TO_X2APIC_MODE, can
be used to instruct Xen to initialize vlapic and do this switch.

Yes, it is another option: Xen can do this switch when need. This
solution leads to smaller code change compared with introducing a new
flag when setting the maximum number of vCPUs.


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