linux-kernel - Re: [PATCH -mm 2/2] kexec/i386: kexec page table code clean up

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Message-ID: <20080110072117.GB26821@verge.net.au>
Date:	Thu, 10 Jan 2008 16:21:17 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	"Huang, Ying" <ying.huang@...el.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Magnus Damm <magnus.damn@...il.com>,
	Ian Campbell <Ian.Campbell@...source.com>
Subject: Re: [PATCH -mm 2/2] kexec/i386: kexec page table code clean up -
	page table setup in C

[ CCing Ian Campbell who handles much of the maintenance of kexec in Xen ]

On Thu, Jan 10, 2008 at 10:08:09AM +0800, Huang, Ying wrote:
> On Wed, 2008-01-09 at 20:05 -0500, Vivek Goyal wrote:
> > On Wed, Jan 09, 2008 at 10:57:50AM +0800, Huang, Ying wrote:
> > > This patch transforms the kexec page tables setup code from asseumbler
> > > code to iC code in machine_kexec_prepare. This improves readability and
> > > reduces code line number.
> > > 
> > 
> > I think this will create issues for Xen. Initially page table setup
> > was in C but Xen Guests could not modify the page tables. I think Xen
> > folks implemented a hypercall where they passed all the page table pages
> > and the control pages and then hypervisor executed the control page(which
> > in turn setup the page tables). I think that's why page table setup
> > code is on the control page in assembly.
> > 
> > You might want to go through Xen kexec implementation and dig through
> > kexec mailing list archive.
> 
> OK, I will check the Xen kexec implementation.
> 
> > CCing Magnus and Horms. They had done the page tables related changes
> > for Xen.

I think that potentially there is a problem here, though weather or
not it manifests is another question.

In machine_kexec() a PAGE_SIZE blob of code starting at relocate_kernel
gets saved. I think that previously x86 Linux excuded this saved blob,
though the current code seems to execute relocate_kernel() directly -
then again perhaps I'm confusing things with ia64 which still executes
the saved blob.

In any case, in the case of xen, the hypervisor will execute this saved
blob. So I think that the crux of the issue is that the blob contains
all the instructions required to run relocate_kernel, then it should
work from inside xen, if and if it doesn't, then i'll blow up.

The code that you have seems to satisfy this requirement,
so relocate_kernel itself shouldn't be a problem. And as
long as the preparation work does exacltly what the removed
portions of relocate_kernel used to do, which it seems to,
things should be fine.

That said, this certainly warrants testing :-)

I'm all in favour of this kind of consolodation,
so hopefully we can bend Xen's will if it doesn't work as is.

Speaking of which, I strongly suspect machine_kexec_32.c and
machine_kexec_64.c an be consolidated a bit.

-- 
Horms

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