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Date: Tue, 21 Mar 2023 12:53:34 +0100
From: "Gerd Hoffmann" <kraxel@redhat.com>
To: "Ni, Ray" <ray.ni@intel.com>
Cc: "devel@edk2.groups.io" <devel@edk2.groups.io>,
	Pawel Polawski <ppolawsk@redhat.com>,
	"Kumar, Rahul R" <rahul.r.kumar@intel.com>,
	Oliver Steffen <osteffen@redhat.com>,
	"Dong, Eric" <eric.dong@intel.com>
Subject: Re: [edk2-devel] [PATCH 1/1] UefiCpuPkg/MpInitLib: fix apic mode for cpu hotplug
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On Tue, Mar 21, 2023 at 07:28:44AM +0000, Ni, Ray wrote:
> > +  } else if ((PcdGet32 (PcdCpuBootLogicalProcessorNumber) > 0) &&
> > +             (PcdGet32 (PcdCpuMaxLogicalProcessorNumber) > 255))
> > +  {
> > +    X2Apic = TRUE;
> 
> Gerd,
> I agree with your needs that want X2 APIC even the actual processor number in BIOS phase <= 255.
> 
> Question: Is it possible that BIOS stays at XAPIC, and later OS switches to X2 APIC when the maximum CPU count > 255?

Linux doesn't do that.  In case x2apic mode is not active at boot the
number of CPUs will be limited to 255.

> I am thinking about adding a new PCD to tell MP code switch to x2 apic
> in the first time AP wakes up. Possible timeline for the code change
> is about within 1 month. Do you think it can meet your needs?

So the idea being that OVMF simply sets that PCD if needed (probably at
the same place where PcdCpuMaxLogicalProcessorNumber is set)?  That
would work too.

take care,
  Gerd