linux-kernel - Re: [PATCH 2/4] x86,xen: introduce x86_init.mapping.pagetable

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Message-ID: <alpine.DEB.2.00.1104141147210.22672@kaball-desktop>
Date:	Thu, 14 Apr 2011 12:05:42 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	Yinghai Lu <yinghai@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"jeremy@...p.org" <jeremy@...p.org>,
	"mingo@...e.hu" <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 2/4] x86,xen: introduce
 x86_init.mapping.pagetable_reserve

On Wed, 13 Apr 2011, H. Peter Anvin wrote:
> On 04/13/2011 03:35 AM, Stefano Stabellini wrote:
> > Yes, that would be ideal, but we cannot do that because we don't know
> > exactly where is pgt_buf_end before allocating the pagetable pages and
> > the pagetable pages need to be marked RO before being hooked into the
> > pagetable. This is why we mark the whole range RO and after the
> > pagetable allocation when we know for sure where is pgt_buf_end we
> > modify the range pgt_buf_end-pgt_buf_top to RW.
> 
> The hell?  You have to fill the pages before you hook them into the page
> tables anyway (this means writing!) and then you have to mark them RO as
> you add them to the page tables... anything else doesn't make any sense
> at all.

Right.
The problem is that at some point init_memory_mapping is going reach the
pagetable pages area and map those pages too (I don't mean hooking the
pagetable pages in the pagetable, I mean mapping them as normal memory
that falls in the range of addresses passed to init_memory_mapping as
argument).
Some of those pages are already pagetable pages (they are in the range
pgt_buf_start-pgt_buf_end) therefore they are going to be mapped RO and
everything is fine.
Some of these pages are not pagetable pages yet (they fall in the range
pgt_buf_end-pgt_buf_top; for example the page at pgt_buf_end) so they
are going to be mapped RW.  When these pages become pagetable pages and
are hooked into the pagetable, xen will find that the guest has already
a RW mapping of them somewhere and fail the operation.

In order to fix the issue I could mark all the pages in the entire range
pgt_buf_start-pgt_buf_top as RO, but then once the pagetable allocation
is completed only the range pgt_buf_start-pgt_buf_end is reserved by
init_memory_mapping therefore the kernel is going to crash as soon as
one of the pages in the range pgt_buf_end-pgt_buf_top is reused.

Initially I suggested to add two hooks: one to allocate the pagetable
pages memory and one to reserve the pagetable pages memory after the
allocation:

http://marc.info/?l=linux-kernel&m=130141955626268

Following Yinghai's suggestion I removed the first hook (currently
unnecessary because we would use the same implementation on native and
on xen) and modified the second one, that became
x86_init.mapping.pagetable_reserve.
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