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Re: [Xen-devel] Re: [Xen-users] [XCP]: RC1 of XCP 0.5 available for test

To: George Shuklin <nge@xxxxxxxx>, Dave Scott <Dave.Scott@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [Xen-users] [XCP]: RC1 of XCP 0.5 available for testing
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Tue, 15 Jun 2010 11:18:35 +0100
Cc: "'xen-devel@xxxxxxxxxxxxxxxxxxx'" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "'xen-users@xxxxxxxxxxxxxxxxxxx'" <xen-users@xxxxxxxxxxxxxxxxxxx>
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On 14/06/2010 15:22, "George Shuklin" <nge@xxxxxxxx> wrote:

> static unsigned long minimum_target(void)
> ...
>         if (max_pfn < MB2PAGES(128))
>                 min_pages = MB2PAGES(8) + (max_pfn >> 1);
>         else if (max_pfn < MB2PAGES(512))
>                 min_pages = MB2PAGES(40) + (max_pfn >> 2);
>         else if (max_pfn < MB2PAGES(2048))
>                 min_pages = MB2PAGES(104) + (max_pfn >> 3);
>         else
>                 min_pages = MB2PAGES(296) + (max_pfn >> 5);
> ...
> I think, those numbers are unreasonable (especially around 2Gb limit).

I never thought putting policy like that in the kernel driver was a good
idea. Its intention is to stop stupid users echoing unrealistic targets into
/proc/xen/balloon. Possibly these checks could be removed, made configurable
on the kernel command line, or only applied to target changes via /proc,
leaving updates via xenstore unchecked.

There's a whole very unproductive argument around this as to whether policy
belongs in a kernel (hard to change, but maybe knows more about the VM),
versus in a higher-level toolstack (easier to change, maybe knows less about
the VM); safety vs flexibility; blah blah blah.

 -- Keir

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