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

To: "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 16:47:29 +0000
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>>> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 16.11.09 17:38 >>>
>Perhaps an extra mechanism in the balloon driver in the Dom0 kernel
>to only give back to Xen pages below the 4GB mark? And if it
>fails return a failure back to Xend?
>That would solve the problem of ballooning out more than needed and
>would eliminate the "oh-lets balloon out 2 more MBs and see if that
>did it".

Unfortunately that's something you won't easily get to work, as you can't
tell Linux' page allocator that you want it to try to allocate a specific
(preselected by you, i.e. the balloon driver) page. But that would be
necessary here, as you would have to scan the p2m table to find suitable

>> >Since the SWIOTLB size is determined by the 'swiotlb' argument passed to
>> >the guest, what if we scanned for that and if it has a number, calculate
>> >how much memory that amounts to and use that value? The default still being 
>> >at 64MB.
>> That might be an option, but is very Linux-centric. I think the amount should
>> be configurable per guest if something like this is being done at all.
>How about both? Have a 'pci_mem' argument to set the default and also
>check the swiotlb argument in 'extra.' If there is a 'swiotlb' argument in
>'extra' use that value (computed for kilobytes of course).

Oh, yes, I was thinking of one augmenting the other, of course.


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