linux-kernel - Re: [Xen-devel] [PATCH]: Fix Xen domU boot with batched mprotect

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Date:	Wed, 15 Oct 2008 16:24:29 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Chris Lalancette" <clalance@...hat.com>
Cc:	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH]: Fix Xen domU boot with batched
	 mprotect

>>> Chris Lalancette <clalance@...hat.com> 15.10.08 13:03 >>>
>The right thing to do is to use arbitrary_virt_to_machine, so that we can be
>sure we are modifying the right pfn.  This unfortunately introduces a
>performance penalty because of a full page-table-walk, but we can avoid that
>penalty for pages in the p2m list by checking if virt_addr_valid is true, and if
>so, just doing the lookup in the p2m table.

Could you explain how virt_addr_valid() can validly be used here? Looking
at its implementation

#define virt_addr_valid(kaddr)	pfn_valid(__pa(kaddr) >> PAGE_SHIFT)

a kaddr in kmap space (i.e. above high_memory) would return a bogus
physical address, and hence pfn_valid() on the resulting pfn is meaningless.

I'd instead simply compare the address in question against high_memory,
and perhaps instead of in arbitrary_virt_to_machine() in
ptep_modify_prot_commit() under an #ifdef CONFIG_HIGHPTE. But
performance-wise, CONFIG_HIGHPTE sucks under Xen anyway, so you'd
better not turn this on in the first place. We may want/need to provide
a means to disable this at run time so the same kernel when run natively
could still make use of it, but without impacting performance under Xen.

Jan

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