linux-kernel - Re: [Xen-devel] Re: [PATCH 00 of 36] x86/paravirt: groundwork for 64-bit Xen support

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Message-ID: <C48859A2.231C5%keir.fraser@eu.citrix.com>
Date:	Wed, 25 Jun 2008 20:22:58 +0100
From:	Keir Fraser <keir.fraser@...citrix.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Mark McLoughlin <markmc@...hat.com>,
	xen-devel <xen-devel@...ts.xensource.com>,
	Eduardo Habkost <ehabkost@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Stephen Tweedie <sct@...hat.com>
Subject: Re: [Xen-devel] Re: [PATCH 00 of 36] x86/paravirt: groundwork for
 64-bit Xen support

On 25/6/08 20:13, "Andi Kleen" <andi@...stfloor.org> wrote:

>> What does Linux expect to scale up to? Reserving 16 PML4 entries leaves the
>> kernel with 120TB of available 'negative' address space. Should be plenty, I
>> would think.
> 
> There are already (ok non x86-64) systems shipping today with 10+TB of
> addressable memory. 100+TB is not that far away with typical
> growth rates. Besides there has to be much more in the negative address
> space than just direct mapping.

There are obviously no x64 boxes around at the moment with >1TB of regular
shared memory, since no CPUs have more than 40 address lines. 100+TB RAM is
surely years away.

If this is a blocker issue, we could just keep PAGE_OFFSET as it is when Xen
support is not configured into the kernel. Then those who are concerned
about 5% extra headroom at 100TB RAM sizes can configure their kernel
appropriately.

> So far we always that 64bit Linux can support upto 1/4*max VA memory.
> With your change that formula would be not true anymore.

Does the formula have any practical significance?

 -- Keir


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