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Re: [Xen-devel] [PATCH Xen-unstable] Balloon down memory to achive enoug

To: <keir.fraser@xxxxxxxxxxxxx>, "Konrad Rzeszutek Wilk" <konrad.wilk@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH Xen-unstable] Balloon down memory to achive enough DMA32 memory for PV guests with PCI pass-through to succesfully launch.
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Mon, 16 Nov 2009 10:02:39 +0000
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>>> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 13.11.09 23:16 >>>
>Balloon down memory to achive enough DMA32 memory for PV guests with PCI 
>pass-through to succesfully launch.
>If the user hasn't used dom0_mem= bootup parameter, the privileged domain
>usurps all of the memory. During launch of PV guests with PCI pass-through
>we ratchet down the memory for the privileged domain to the required memory
>for the PV guest. However, for PV guests with PCI pass-through we do not
>take into account that the PV guest is going to swap its SWIOTLB memory
>for DMA32 memory - in fact, swap 64MB of it. This patch balloon's down
>the privileged domain so that there are 64MB of DMA32 memory available.

While I realize that the patch was already committed, I still wanted to
raise my concerns with it:

For one, it is logically (just for a much smaller total amount and for a more
narrow memory range) identical to what would be needed for 32-bit-pv
DomU-s on 64-bit hv, so *if* this patch is considered conceptually valid,
then it should be abstracted and used for both purposes.

I think, however, that it is conceptually wrong, because it may mean that
all of the memory possibly removable from Dom0 can get ballooned out
without in fact yielding the memory needed by the to be started DomU.

Besides that, hard-coding the value to 64Mb doesn't seem very nice
either (while I realize that both 2.6.18 and pv-ops default to 64Mb, I
do not think this is really appropriate, especially given that in the 2.6.18
tree Dom0 can get run with as little as 2Mb, and I highly doubt that the
demand of a DomU can by default be assumed to exceed that of Dom0),
and in particular doesn't help with the case where one really has to use
a larger than the default size swiotlb.


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