From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: Xen Hypervisor's memory get roughly less 700MB(even
	1400MB) memory than actual grub setting when >4GB
Date: Wed, 01 Dec 2010 00:25:57 -0800
Message-ID: <4CF60695.7080303@goop.org>
References: <AANLkTikTvQazS54xKHRUGAg91D6AJc=zAVY9kt42E__R@mail.gmail.com>
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To: benian <bestwish.happiness@gmail.com>
Cc: xen-devel@lists.xensource.com, Ian Jackson <Ian.Jackson@eu.citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>
List-Id: xen-devel@lists.xenproject.org

On 12/01/2010 12:00 AM, benian wrote:
>
> Hi,
>
> I am using Xen 4.0.1-6 and fedora2.6.32.23-170.xendom0.fc12.x86_64
>
> =20
>
> When I try to set the a fix value for memory on grub
>
> =20
>
> For example
>
> =20
>
> title Xen_Fedora (2.6.32.23-170.xendom0.fc12.x86_64)
>
>         root (hd0,0)
>
>         kernel /xen-4.0.1.gz dom0_mem=3D5120M
>
> =20
>
> I found that when I set dom0_mem<=3D3GB, the system memory report by
> =93free=94 =93xm list=94 =93xl list=94 and =93xentop=94 are both approx=
imately conform
> with the setting in grub.
>
> But when dom_mem>=3D4GB(roughly), the system memory is obviously
> different from the grub.
>

Yes, that's expected.  The BIOS reserves a chunk of address space
between 3-4G to map PCI devices into.  Since dom0 needs to get access to
those devices, we free the Xen-supplied memory in that range back to the
hypervisor.  You should see that reflected in the free memory output of
"xl info".

In recent versions of xen/stable-2.6.32.x you can balloon dom0 back up
again to its full size with "xl mem-set 0 <size>", but I don't think
that's implemented in the version you're using.

> =93xm list=94
>
> Name                 ID   Mem VCPUs      State   Time(s)
>
> Domain-0              0 * 4384*     8     r-----    136.8
>
> =20
>
> =20
>
> =20
>
> =93xl list=94
>
> Name                 ID   Mem VCPUs       State        Time(s)
>
> Domain-0              0  *3648*     8        r--    139.8
>

That's just strange: xl and xm should show the same results here.

    J