linux-kernel - Re: [STABLE REGRESSION] Possible missing backport of x86_match

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Message-ID: <05ced22b5b68e338795c8937abb8141d9fa188e6.camel@intel.com>
Date: Wed, 18 Sep 2024 06:54:33 +0000
From: "Zhang, Rui" <rui.zhang@...el.com>
To: "regressions@...mhuis.info" <regressions@...mhuis.info>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC: "Neri, Ricardo" <ricardo.neri@...el.com>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "bp@...en8.de" <bp@...en8.de>, "Gupta, Pawan
 Kumar" <pawan.kumar.gupta@...el.com>, "regressions@...ts.linux.dev"
	<regressions@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Luck, Tony" <tony.luck@...el.com>,
	"thomas.lindroth@...il.com" <thomas.lindroth@...il.com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [STABLE REGRESSION] Possible missing backport of x86_match_cpu()
 change in v6.1.96

On Mon, 2024-08-12 at 14:11 +0200, Greg KH wrote:
> On Wed, Aug 07, 2024 at 10:15:23AM +0200, Thorsten Leemhuis wrote:
> > [CCing the x86 folks, Greg, and the regressions list]
> > 
> > Hi, Thorsten here, the Linux kernel's regression tracker.
> > 
> > On 30.07.24 18:41, Thomas Lindroth wrote:
> > > I upgraded from kernel 6.1.94 to 6.1.99 on one of my machines and
> > > noticed that
> > > the dmesg line "Incomplete global flushes, disabling PCID" had
> > > disappeared from
> > > the log.
> > 
> > Thomas, thx for the report. FWIW, mainline developers like the x86
> > folks
> > or Tony are free to focus on mainline and leave stable/longterm
> > series
> > to other people -- some nevertheless help out regularly or
> > occasionally.
> > So with a bit of luck this mail will make one of them care enough
> > to
> > provide a 6.1 version of what you afaics called the "existing fix"
> > in
> > mainline (2eda374e883ad2 ("x86/mm: Switch to new Intel CPU model
> > defines") [v6.10-rc1]) that seems to be missing in 6.1.y. But if
> > not I
> > suspect it might be up to you to prepare and submit a 6.1.y variant
> > of
> > that fix, as you seem to care and are able to test the patch.
> 
> Needs to go to 6.6.y first, right?  But even then, it does not apply
> to
> 6.1.y cleanly, so someone needs to send a backported (and tested)
> series
> to us at stable@...r.kernel.org and we will be glad to queue them up
> then.
> 
> thanks,
> 
> greg k-h

There are three commits involved.

commit A:
   4db64279bc2b (""x86/cpu: Switch to new Intel CPU model defines"") 
   This commit replaces
      X86_MATCH_INTEL_FAM6_MODEL(ANY, 1),             /* SNC */
   with
      X86_MATCH_VFM(INTEL_ANY,         1),    /* SNC */
   This is a functional change because the family info is replaced with
0. And this exposes a x86_match_cpu() problem that it breaks when the
vendor/family/model/stepping/feature fields are all zeros.

commit B:
   93022482b294 ("x86/cpu: Fix x86_match_cpu() to match just
X86_VENDOR_INTEL")
   It addresses the x86_match_cpu() problem by introducing a valid flag
and set the flag in the Intel CPU model defines.
   This fixes commit A, but it actually breaks the x86_cpu_id
structures that are constructed without using the Intel CPU model
defines, like arch/x86/mm/init.c.

commit C:
   2eda374e883a ("x86/mm: Switch to new Intel CPU model defines")
   arch/x86/mm/init.c: broke by commit B but fixed by using the new
Intel CPU model defines

In 6.1.99,
commit A is missing
commit B is there
commit C is missing

In 6.6.50,
commit A is missing
commit B is there
commit C is missing

Now we can fix the problem in stable kernel, by converting
arch/x86/mm/init.c to use the CPU model defines (even the old style
ones). But before that, I'm wondering if we need to backport commit B
in 6.1 and 6.6 stable kernel because only commit A can expose this
problem.

thanks,
rui