linux-kernel - Re: [PATCH 1/1] x86/pci: Skip early E820 check for ECAM region

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Date: Wed, 17 Apr 2024 16:09:43 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: linux-pci@...r.kernel.org, Mateusz Kaduk <mateusz.kaduk@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, Tj <linux@....tj>,
	Hans de Goede <hdegoede@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/1] x86/pci: Skip early E820 check for ECAM region

On Wed, Apr 17, 2024 at 11:47:27PM +0300, Andy Shevchenko wrote:
> On Wed, Apr 17, 2024 at 11:40 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > Reported-by: Mateusz Kaduk <mateusz.kaduk@...il.com>
> 
> > Reported-by: Arul <...>
> > Reported-by: Imcarneiro91 <...>
> > Reported-by: Aman <...>
> 
> Isn't bugzilla public enough? You may take emails from there, no?

Good question.  I think email addresses in bugzilla are only visible
to logged-in users, so I wanted permission before publishing them.  I
got that from Mateusz and am hoping to hear from the others because I
want everybody to get credit for their contribution.

> > +               /*
> > +                * 946f2ee5c731 ("Check that MCFG points to an e820
> > +                * reserved area") added this E820 check in 2006 to work
> > +                * around BIOS defects.
> > +                *
> > +                * Per PCI Firmware r3.3, sec 4.1.2, ECAM space must be
> > +                * reserved by a PNP0C02 resource, but it need not be
> > +                * mentioned in E820.  Before the ACPI interpreter is
> > +                * available, we can't check for PNP0C02 resources, so
> > +                * there's no reliable way to verify the region in this
> > +                * early check.  Keep it only for the old machines that
> > +                * motivated 946f2ee5c731.
> > +                */
> 
> > +               if (dmi_get_bios_year() < 2016 && raw_pci_ops)
> 
> I probably missed something, but where does 2016 come from?
> (I've been following the bz discussion)

I made it up based on the fact that 946f2ee5c731 was added in 2006,
and I just added 10 years.  I would love to get rid of the E820 checks
altogether because they're really completely bogus and an ongoing
headache, but I don't know the details of the machines that
946f2ee5c731 fixed.  I'm open to other suggestions.  The Lenovo BIOS
is from 2023, so it would have to be something earlier than that:

  DMI: LENOVO 83AG/LNVNB161216, BIOS MHCN40WW 12/15/2023

> >         if (pci_mmcfg_running_state)
> >                 return true;
> >
> > -       /* Don't try to do this check unless configuration
> > -          type 1 is available. how about type 2 ?*/
> > -       if (raw_pci_ops)
> > -               return is_mmconf_reserved(e820__mapped_all, cfg, dev,
> > -                                         "E820 entry");
> > -
> >         return false;
> 
> Not strictly related to this patch, but now it can simply
> 
>   return pci_mmcfg_running_state;

Good point, changed locally.

> In any case, LGTM,
> Reviewed-by: Andy Shevchenko <andy@...nel.org>

Thanks!

Bjorn