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[Xen-devel] Re: [PATCH 2/4] x86,xen: introduce x86_init.mapping.pagetabl

To: "H. Peter Anvin" <hpa@xxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 2/4] x86,xen: introduce x86_init.mapping.pagetable_reserve
From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date: Thu, 14 Apr 2011 19:09:47 +0100
Cc: "jeremy@xxxxxxxx" <jeremy@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "konrad.wilk@xxxxxxxxxx" <konrad.wilk@xxxxxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, "mingo@xxxxxxx" <mingo@xxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxxxxxxxx>, "yinghai@xxxxxxxxxx" <yinghai@xxxxxxxxxx>
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On Thu, 14 Apr 2011, H. Peter Anvin wrote:
> On 04/14/2011 04:30 AM, Stefano Stabellini wrote:
> > 
> > I hope that the other email I have just sent clarifies the purpose of
> > the hook.
> >             
> You're commingling rationale and (proposed) semantics.  BOTH need to be
> stated clearly.

Sorry I didn't answer the right question straight away.
The semantics follow:

It is as if there is an implicit pvop after find_early_table_space
that exports the range pgt_buf_start - pgt_buf_top.
The range indicated is used, and it is the only range used, for the initial
kernel pagetable pages.
The range indicated is an overestimate.
It is implicit because we use the pgt_buf_* variables directly but
nonetheless it was in the first version of the patch.

After the pagetable setup done by kernel_physical_mapping_init, we know
exactly how many pages we actually used for the initial kernel
pagetables and how many are left unused.

The purpose of the second pvop (x86_init.mapping.pagetable_reserve) is
to notify sub-architectures of the actual range used for initial kernel
pagetable pages and partially revoke the previous indication.

If you agree with the proposed semantics above, I'll add the description
to the commit message as well as the code. I could also modify the
syntax of the new pvop call to better suit the semantics and/or restore
the first pvop call.

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