linux-kernel - Re: [PATCH v2 14/16] x86,nvdimm,kexec: Use walk_iomem_res

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Date:	Mon, 4 Jan 2016 19:04:27 +0800
From:	Dave Young <dyoung@...hat.com>
To:	Toshi Kani <toshi.kani@....com>
Cc:	akpm@...ux-foundation.org, bp@...en8.de,
	linux-arch@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Dan Williams <dan.j.williams@...el.com>, x86@...nel.org,
	linux-nvdimm@...1.01.org, kexec@...ts.infradead.org
Subject: Re: [PATCH v2 14/16] x86,nvdimm,kexec: Use walk_iomem_res_desc() for
 iomem search

On 01/04/16 at 05:25pm, Dave Young wrote:
> Hi, Toshi,
> 
> On 12/25/15 at 03:09pm, Toshi Kani wrote:
> > Change to call walk_iomem_res_desc() for searching resource entries
> > with the following names:
> >  "ACPI Tables"
> >  "ACPI Non-volatile Storage"
> >  "Persistent Memory (legacy)"
> >  "Crash kernel"
> > 
> > Note, the caller of walk_iomem_res() with "GART" is left unchanged
> > because this entry may be initialized by out-of-tree drivers, which
> > do not have 'desc' set to IORES_DESC_GART.
> 
> Found below commit which initialize the GART entry:
> commit 56dd669a138c40ea6cdae487f233430d12372767
> Author: Aaron Durbin <adurbin@...gle.com>
> Date:   Tue Sep 26 10:52:40 2006 +0200
> 
>     [PATCH] Insert GART region into resource map
>     
>     Patch inserts the GART region into the iomem resource map. The GART will then
>     be visible within /proc/iomem. It will also allow for other users
>     utilizing the GART to subreserve the region (agp or IOMMU).
>     
>     Signed-off-by: Aaron Durbin <adurbin@...gle.com>
> 
> But later it was reverted:
> commit 707d4eefbdb31f8e588277157056b0ce637d6c68
> Author: Bjorn Helgaas <bhelgaas@...gle.com>
> Date:   Tue Mar 18 14:26:12 2014 -0600
> 
>     Revert "[PATCH] Insert GART region into resource map"
>     
>     This reverts commit 56dd669a138c, which makes the GART visible in
>     /proc/iomem.  This fixes a regression: e501b3d87f00 ("agp: Support 64-bit
>     APBASE") exposed an existing problem with a conflict between the GART
>     region and a PCI BAR region.
>     
>     The GART addresses are bus addresses, not CPU addresses, and therefore
>     should not be inserted in iomem_resource.
>     
>     On many machines, the GART region is addressable by the CPU as well as by
>     an AGP master, but CPU addressability is not required by the spec.  On some
>     of these machines, the GART is mapped by a PCI BAR, and in that case, the
>     PCI core automatically inserts it into iomem_resource, just as it does for
>     all BARs.
>     
>     Inserting it here means we'll have a conflict if the PCI core later tries
>     to claim the GART region, so let's drop the insertion here.
>     
>     The conflict indirectly causes X failures, as reported by Jouni in the
>     bugzilla below.  We detected the conflict even before e501b3d87f00, but
>     after it the AGP code (fix_northbridge()) uses the PCI resource (which is
>     zeroed because of the conflict) instead of reading the BAR again.
>     
>     Conflicts:
>         arch/x86_64/kernel/aperture.c
>     
>     Fixes: e501b3d87f00 agp: Support 64-bit APBASE
>     Link: https://bugzilla.kernel.org/show_bug.cgi?id=72201
>     Reported-and-tested-by: Jouni Mettälä <jtmettala@...il.com>
>     Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
> 
> 
> For amd64 agp, currently the region name is "aperture" instead:
> drivers/char/agp/amd64-agp.c: agp_aperture_valid()
> 
> This may not be the only case, but I doubt that anyone is testing this since
> long time ago kexec-tools excluding the 'GART' region. Kexec-tools and kexec_file
> may need update to use "aperture" if someone can test it.
> 
> I think adding an enum value for compatibility is reasonable, we do not care
> about third party drivers in mainline.

Hmm, rethink about it, the new kernel will not export "GART" regions thus the in-kernel loader should never see such regions as long as there's no other drivers exporting it. Thus dropping the code chunk in crash.c about 'GART' should be fine.

Thanks
Dave
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