linux-kernel - Re: How could we get rid of saved_max

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Date:	Wed, 19 Feb 2014 17:04:22 -0700
From:	Jon Mason <jdmason@...zu.us>
To:	WANG Chao <chaowang@...hat.com>
Cc:	Muli Ben-Yehuda <muli@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Vivek Goyal <vgoyal@...hat.com>,
	Yinghai Lu <yinghai@...nel.org>, kexec@...ts.infradead.org,
	discuss@...-64.org, x86@...nel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: How could we get rid of saved_max_pfn for calgary iommu?

On Tue, Feb 18, 2014 at 11:18 PM, WANG Chao <chaowang@...hat.com> wrote:
> Hi, All
>
> arch/x86/kernel/pci-calgary.c is the only user of saved_max_pfn today:
>
> int __init detect_calgary(void)
> {
>         [..]
>         specified_table_size = determine_tce_table_size((is_kdump_kernel() ?
>                                         saved_max_pfn : max_pfn) * PAGE_SIZE);
>         [..]
> }

IIUC, the purpose of this code is to reuse the TCE table from the
previous kernel.  Thus, it needs to be of the same size as the
pre-kdump kernel.  It is using the max_pfn to determine the TCE table
size in the non-kdump case.  If there is another way to determine the
size it used before, then I am fine making the change to use that way.

Thanks,
Jon

> saved_max_pfn is the real mem size and is calculated by 1st kernel E820
> memmap which is passed in by 2nd kernel's boot_params (done by kexec):
>
>         saved_max_pfn = e820_end_of_ram_pfn();
>
> After saved_max_pfn has been set, memmap=exactmap will reset the E820
> provided by boot_params and use the user defined E820 instead.
>
> Now we want to get rid of memmap=exactmap and directly pass the E820
> memmap by boot_params for some reason (eg. exactmap may exceed the cmdline
> size and also isn't compatible with kaslr).
>
> However saved_max_pfn becomes the obstacle for obsoleting exactmap.
> Because it needs two conditions: first kernel's E820 map and
> memmap=exactmap cmdline.
>
> So I'm wondering if it's possible to get rid of saved_max_pfn totally in
> calgary code. Or we can get saved_max_pfn using a different way, for
> example calculated in 1st kernel and passed in to 2nd kernel by cmdline.
>
> Thanks
> WANG Chao
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