linux-kernel - Re: [PATCH 0/2] xen: Switch to virtual mapped linear p2m list

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Message-ID: <544F22E4.9040706@suse.com>
Date:	Tue, 28 Oct 2014 06:00:20 +0100
From:	Juergen Gross <jgross@...e.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH 0/2] xen: Switch to virtual mapped linear p2m list

On 10/27/2014 04:16 PM, David Vrabel wrote:
> On 27/10/14 14:52, Juergen Gross wrote:
>> Paravirtualized kernels running on Xen use a three level tree for
>> translation of guest specific physical addresses to machine global
>> addresses. This p2m tree is used for construction of page table
>> entries, so the p2m tree walk is performance critical.
>>
>> By using a linear virtual mapped p2m list accesses to p2m elements
>> can be sped up while even simplifying code. To achieve this goal
>> some p2m related initializations have to be performed later in the
>> boot process, as the final p2m list can be set up only after basic
>> memory management functions are available.
>
> What impact does this have on 32-bit guests which don't have huge amount
> of virtual address space?
>
> I think a 32-bit guest could have up to 64 GiB of PFNs, which would
> require a 128 MiB p2m array, which is too large?

It is 64 MB (one entry on 32 bit is 32 bits :-) ).

With a m2p array of only 16 MB size I doubt a 32 bit guest can be larger
than 16 GB, or am I wrong here?


Juergen
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