linux-kernel - Re: [Xen-devel] [PATCH 1/9] xen: make sure swiotlb allocation isphysically contigious

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Date:	Fri, 08 May 2009 08:47:00 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Jeremy Fitzhardinge" <jeremy.fitzhardinge@...rix.com>,
	"Ingo Molnar" <mingo@...e.hu>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	"FUJITA Tomonori" <fujita.tomonori@....ntt.co.jp>,
	"Xen-devel" <xen-devel@...ts.xensource.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 1/9] xen: make sure swiotlb allocation
	 isphysically contigious

>>> Jeremy Fitzhardinge <jeremy@...p.org> 08.05.09 02:17 >>>
>Impact: make swiotlb allocation suitable for Xen
>
>When allocating the swiotlb buffer under Xen, make sure the memory is
>physically contiguous so that its really suitable for DMA.
>
>Do this by allocating the memory as usual, but then call a Xen
>function to rearrange the underlying pages to be physically
>contiguous.

As pointed out before, you must not use alloc_bootmem_low...() here,
as on large systems you may otherwise get an allocation failure (i.e. a
panic - checking the return value is pointless). The alternative is to fix
the bootmem allocator to no longer allocate from bottom up.

Jan

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