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[Xen-devel] Re: [Patch] by default don't give all memory to dom0

To: "Siddha, Suresh B" <suresh.b.siddha@xxxxxxxxx>
Subject: [Xen-devel] Re: [Patch] by default don't give all memory to dom0
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Fri, 19 Aug 2005 09:44:03 +0100
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On 18 Aug 2005, at 20:24, Siddha, Suresh B wrote:

By default, xen needs to reserve some portion of memory to satisfy
SWIOTLB and other contguous memory region requests. Following
the current swiotlb enabling mechanism, Appended patch reserves 128MB
of memory on systems with more than 2GB of RAM.

Hmmm... sounds reasonable. I'd rather have one dom0 memory parameter though --- keeping dom0_mem but have +ve value mean 'allocate this amount' and -ve mean 'allocate full memory - this amount'. Otherwise we have two competing parameters specifying basically the same thing...

I don't much like hacky 'policies' that hardcode default reservations in the hypervisor, but I think this one is pretty sensible.

Ideally shouldn't we enable SWIOTLB in dom0 and this DMA memory reservation in hypervisor by default? Otherwise we will have a problem(even on systems
with less than 2GB of RAM) in servicing a driver DMA request to a
kmalloc'd buffer which spans more than a page or the various
xen_create_contiguous_region() requests.

You get a pretty clear BUG() message out if this happens. It actually tells you to enable 'swiotlb=force'! The number of drivers that actually use multi-page buffers is really small -- we only fixed that case in 2.0 a few weeks ago.

I'd rather not waste 64MB of pre-reserved bounce buffers on small-memory systems that almost certainly don't need bounce buffers.

 -- Keir

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