linux-kernel - How could we get rid of saved_max

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Message-ID: <20140219061824.GA29703@dhcp-17-89.nay.redhat.com>
Date:	Wed, 19 Feb 2014 14:18:24 +0800
From:	WANG Chao <chaowang@...hat.com>
To:	Muli Ben-Yehuda <muli@...ibm.com>,
	"Jon D. Mason" <jdmason@...zu.us>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Vivek Goyal <vgoyal@...hat.com>,
	Yinghai Lu <yinghai@...nel.org>
Cc:	kexec@...ts.infradead.org, discuss@...-64.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: How could we get rid of saved_max_pfn for calgary iommu?

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);
	[..]
}

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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