Message-ID: <20171112080426.GB6429@x1>
Date:   Sun, 12 Nov 2017 16:04:26 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Jiri Bohac <jbohac@...e.cz>, yinghai@...nel.org, joro@...tes.org,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     Toshi Kani <toshi.kani@....com>, David Airlie <airlied@...ux.ie>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Young <dyoung@...hat.com>, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] x86/kexec: Exclude GART aperture from vmcore

On 11/07/17 at 04:34pm, Jiri Bohac wrote:
> On Tue, Nov 07, 2017 at 02:42:12PM +0100, Jiri Bohac wrote:
> > On Tue, Nov 07, 2017 at 07:39:56PM +0800, Baoquan He wrote:
> > > don't worry about the user space kexec utility either. 
> > 
> > What's the problem with the userspace kexec? The bug is in
> > reading /proc/vmcore by makedumpfile. kexec would only operate
> > within the preallocated crashkernel area, right?
> 
> right, I see it (without -s the kexec userspace creates the ELF header
> later used the second kernel for /proc/vmcore).

Yes, I meant this. In kernel, you can define global variable to store
the starting address and end of GART aperture. While, in user space
there's no way to know that. Now the non '-s' kexec are still being used
by most of people.

I roughly went through agp3.0 doc and GART code, the root cause for this
issue should be:

AMD system with GART need be enabled in BIOS in principle. Then firmware
will arrange a hole in system address space, defaultly it's 64MB for GART
aperture mapping, below and close to 4G usually. GART stands for Graphic
Address Remap Table, each of its entry can be used to refer to a address
region in the 64M of aperture for iommu usage.

However, in your testing AMD system, you don't enable GART IOMMU support
in BIOS setting. So the current implementation in kernel is to find a
region which is occupied by system RAM and configre the starting addr
and size into GART cofig registers' AMD64_GARTAPERTURECTL and
AMD64_GARTAPERTUREBASE. And this happens in the first kernel. I believe
in kdump kernel, since only resereved crashkernel region is taken as
available system RAM, the rest of original RAM space is seen as hole.
So kdump kernel will still use the 1st kernel's aperture region for GART,
and it also has been set in GART register, kdump kernel think it as BIOS
has reserved hole for GART aperture.

Now the problem is that those pages reserved for GART aperture have been
added into mm subsystem. GART is located on North Bridge. But when CPU
try to access these them, will check North Bridge chip firstly, then
hardware error occured that region has been set in GART registers which
locates in NB.

Solution:
1) Remove the code which support GART IOMMU when it's not enabled in
BIOS. This has been done in the new generation of hardware IOMMU like
intel vt-d IOMMU and amd-Vi IOMMU. We should not make GART IOMMU be
exceptional.

2) Remove those pages from mm subsystem since they are not seen any more
though they have been added into mm subsystem, because CPU can't see
them.

3) Remove the apreture region from /proc/iomem so that pages in that
region can't be seen by kdump kernel. This is easier, but just a work
around.

Hi Yinghai, Joerg, and Bjorn

Found patches you contributed to GART IOMMU, do you have any suggestion
about this issue? Or any comment about these 3 options? 

I personally prefer the 1st one.

Thanks
Baoquan

> 
> No idea how to fix that nicely...
> 
> -- 
> Jiri Bohac <jbohac@...e.cz>
> SUSE Labs, Prague, Czechia
> 
> 
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