linux-kernel - Re: [patch 06/26] Xen-paravirt_ops: paravirt

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Date:	Tue, 27 Feb 2007 16:49:53 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org,
	xen-devel@...ts.xensource.com, Chris Wright <chrisw@...s-sol.org>,
	Zachary Amsden <zach@...are.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [patch 06/26] Xen-paravirt_ops: paravirt_ops: allocate a fixmap
 slot

Jeremy Fitzhardinge wrote:
> Ingo Molnar wrote:
>   
>> why not vmalloc it on the guest side? fixmaps are bad for this purpose 
>> for a general paravirt implementation, it limits the size of the shared 
>> info page, etc.
>>     
>
> Yes.  vmalloc would have the annoying side-effect of actually allocating
> some pages which would be shadowed by the remapping, but I guess
> get_vm_area would do the job.  I'll give it a go.

Hm, this is a bit awkward.  We need to map the shared info page fairly
early - say, around paging_init - but we're still on the bootmem
allocator at that point, so get_vm_area isn't usable yet.  Using a
fixmap keeps things simple.  It seems to me that having a single fixmap
available is useful for this kind of simple/early mapping, and if
someone needs to map something larger, then they can put it off until
get_vm_area() is available...

    J
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