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Re: [Xen-devel] [PATCH 10/27] xen: make sure swiotlb allocation isphysi

To: Jan Beulich <jbeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 10/27] xen: make sure swiotlb allocation isphysically contigious
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 16 Mar 2009 12:37:05 -0700
Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, David Airlie <airlied@xxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>
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Jan Beulich wrote:
While on native using alloc_bootmem_low_pages() is a requirement here,
on Xen this should explicitly not be used, as we realized just a couple of
days ago:

Which conversation was this?

 The way the bootmem allocator works, running out of space
below 4Gb is pretty easy on machines with lots of memory, and since the
swiotlb is a requirement for Dom0, the risk of allocation failures must be
kept as low as possible.

Are we talking about a 32 or 64 bit dom0 here? If its 32-bit, then yes, low memory is a precious resource, but I don't see why that depends on the total amount of memory. And if its 64-bit, why is below-4G particularly constrained? That would only matter for 32-bit devices?


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