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

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Message-ID: <alpine.DEB.2.00.1104141218110.22672@kaball-desktop>
Date:	Thu, 14 Apr 2011 12:30:11 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"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>,
	"yinghai@...nel.org" <yinghai@...nel.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 04/12/2011 04:19 AM, stefano.stabellini@...citrix.com wrote:
> > From: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > 
> > Introduce a new x86_init hook called pagetable_reserve that during the
> > initial memory mapping is used to reserve a range of memory addresses for
> > kernel pagetable usage.
> > 
> > On native it just calls memblock_x86_reserve_range while on xen it also
> > takes care of setting the spare memory previously allocated
> > for kernel pagetable pages from RO to RW, so that it can be used for
> > other purposes.
> > 
> 
> What are the *semantics* of this hook?
> 
> Hooks are insanely nasty if they are just defined by a particular code
> flow, as evidenced by the royal mess called paravirt_ops.

I hope that the other email I have just sent clarifies the purpose of
the hook.
I admit that as it is it wouldn't find much usage outside
init_memory_mapping.
Maybe adding the corresponding hook to allocate the initial kernel
pagetable pages would help generalizing it.  Or maybe we just need a
better comment in the code:


/* Reserve the kernel pagetable pages we used and free the other ones so
 * that they can be reused for other purposes.
 * 
 * On native it just means calling memblock_x86_reserve_range, on Xen it
 * also means marking RW the pagetable pages that we allocated before
 * but that haven't been used here.
 */
if (!after_bootmem && pgt_buf_end > pgt_buf_start)
		x86_init.mapping.pagetable_reserve(PFN_PHYS(pgt_buf_start),
				PFN_PHYS(pgt_buf_end));
		
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