linux-kernel - RE: [patch 19/58] x86/apic: Get rid of apic

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Date:   Fri, 21 Jul 2023 04:17:56 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
CC:     "x86@...nel.org" <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wei Liu <wei.liu@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Juergen Gross <jgross@...e.com>
Subject: RE: [patch 19/58] x86/apic: Get rid of apic_phys

From: Thomas Gleixner <tglx@...utronix.de> Sent: Thursday, July 20, 2023 1:03 AM
> 
> On Thu, Jul 20 2023 at 04:18, Michael Kelley wrote:
> 
> > From: Thomas Gleixner <tglx@...utronix.de> Sent: Tuesday, July 18, 2023 6:12 AM
> >>
> >> On Tue, Jul 18 2023 at 01:15, Thomas Gleixner wrote:
> >> > @@ -1921,7 +1922,6 @@ static __init void try_to_enable_x2apic(
> >> >  		 * be addressed must not be brought online.
> >> >  		 */
> >> >  		x2apic_set_max_apicid(apic_limit);
> >> > -		x2apic_phys = 1;
> 
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This hunk _IS_ wrong and needs to be
> reverted obvioulsy.
> 
> >> >  	}
> >> >  	x2apic_enable();
> >> >  }
> >>
> >> This hunk is obviously bogus. I just noticed on a VM which takes this
> >> code path...
> >
> > I'm testing guests on Hyper-V.  The case where the x2apic is enabled
> > in the BIOS works, but when the x2apic must be enabled by Linux,
> > the VMbus drivers never get initialized and things go downhill from
> > there. Your comment above is somewhat cryptic (as I haven't studied
> > the patches in detail), but I'm guessing it explains the failure I'm seeing.
> >
> > Let me know if I should debug the failure I'm seeing.  Otherwise
> > I'll wait for a new version and try again.
> 
> Can you add that line back and retest?
> 

Everything is looking good now -- my VMbus driver problem was actually
an unrelated config problem.   All combinations of Hyper-V VMs that
I've tried with xapic and x2apic, and on Intel and AMD processors,
are working.   There are couple more old configs that I want to try
tomorrow or over the weekend.

Michael

Michael