linux-kernel - Re: [Xen-devel] [PATCH 5/7] xen/p2m: Add logic to revector a P2M tree to use _

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Date:	Fri, 27 Jul 2012 12:47:47 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Stefano Stabellini" <stefano.stabellini@...citrix.com>
Cc:	"xen-devel" <xen-devel@...ts.xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 5/7] xen/p2m: Add logic to revector a
 P2M tree to use __va leafs.

>>> On 27.07.12 at 13:18, Stefano Stabellini <stefano.stabellini@...citrix.com> wrote:
> On Thu, 26 Jul 2012, Konrad Rzeszutek Wilk wrote:
>>  1) All P2M lookups instead of using the __ka address would
>>     use the __va address. This means we can safely erase from
>>     __ka space the PMD pointers that point to the PFNs for
>>     P2M array and be OK.
>>  2). Allocate a new array, copy the existing P2M into it,
>>     revector the P2M tree to use that, and return the old
>>     P2M to the memory allocate. This has the advantage that
>>     it sets the stage for using XEN_ELF_NOTE_INIT_P2M
>>     feature. That feature allows us to set the exact virtual
>>     address space we want for the P2M - and allows us to
>>     boot as initial domain on large machines.
>> 
>> So we pick option 2).
> 
> 1) looks like a decent option that requires less code.
> Is the problem with 1) that we might want to access the P2M before we
> have __va addresses ready?

AIUI 1) has no easy way of subsequently accommodating support
for XEN_ELF_NOTE_INIT_P2M (where you almost definitely will
want/need to reclaim the originally used VA space - if nothing else,
then for forward compatibility with the rest of the kernel).

Jan

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