linux-kernel - Re: [PATCH] x86_64: allow sections that are recycled to set _PAGE

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Date:	Sat, 13 Feb 2010 21:12:08 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"jeremy@...p.org" <jeremy@...p.org>
Subject: Re: [PATCH] x86_64: allow sections that are recycled to set _PAGE_RW

On 02/13/2010 12:08 PM, Suresh Siddha wrote:
>
> Checks in static_protections() ensure that we map the text mapping as
> read-only (and don't bother about what permissions it uses for
> underlying free pages that get freed in free_init_pages()). But the
> kernel direct mappings for free pages will be RW and ensure that the
> free pages can be read/written using the direct mapping.
>
> The checks in static_protections() for kernel text mapping ensure that
> we don't break the 2MB kernel text pages unnecessarily on 64bit kernels
> (as it has performance implications). We should be fine as long as the
> kernel identity mappings reflect the correct RW permissions.
>
> But somehow this is working fine on native kernels but not on Xen pv
> guest. Your patch will cause the performance issues that we are
> addressing using the static protections checks. I will look at this more
> detailed on tuesday.
>

Xen doesn't support PSE, so it has the (otherwise impossible) 
combination of PAE and !PSE ... i.e. it only uses 4K pages.

	-hpa
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