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

From: Chao Gao <chao.gao@intel.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: "Andrew Cooper" <andrew.cooper3@citrix.com>,
	xen-devel@lists.xen.org, "Wei Liu" <wei.liu2@citrix.com>,
	"Ian Jackson" <ian.jackson@eu.citrix.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [RFC Patch v4 4/8] hvmloader: boot cpu through broadcast
Date: Mon, 26 Feb 2018 20:33:23 +0800	[thread overview]
Message-ID: <20180226123322.GA140947@skl-4s-chao.sh.intel.com> (raw)
In-Reply-To: <5A93D32702000078001AB92A@prv-mh.provo.novell.com>

On Mon, Feb 26, 2018 at 01:28:07AM -0700, Jan Beulich wrote:
>>>> On 24.02.18 at 06:49, <chao.gao@intel.com> 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.

Thanks
Chao

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