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RE: [Xen-devel] [PATCH] Fix auto-ballooning of dom0 for HVM domains

To: "Kevin Tian" <kevin.tian@xxxxxxxxx>, "Yunhong Jiang" <yunhong.jiang@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] [PATCH] Fix auto-ballooning of dom0 for HVM domains
From: "Charles Coffing" <ccoffing@xxxxxxxxxx>
Date: Mon, 22 May 2006 08:51:02 -0600
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>>> On Mon, May 22, 2006 at  2:51 AM, in message
<571ACEFD467F7749BC50E0A98C17CDD8094E7C7E@pdsmsx403>, "Tian, Kevin"
<kevin.tian@xxxxxxxxx> wrote: 
> BTW, could you please explain why following change is required:
> +        given amount, also in KiB.  This is normally just mem, but
> HVM is
> +        supported, keep a little extra free."""
> +        if 'hvm' in xc.xeninfo()['xen_caps']:
> +            mem_kb += 4*1024;
> +        return mem_kb
> Why do you need to reserve extra memory even for domU as long as 
> the platform supports hvm feature? 

HVM domains need some extra memory free for the shadow page tables,
otherwise they may crash the entire machine while they are running, or
the HVM domain itself may crash (exact behavior depends on whether you
have Yunhong's patch to change BUGs to domain_crash).

This slack space is calculated into the memory size for HVM domains,
but what happens if you then start a PV domain afterwards?  Only the
minimally required memory is freed up, then the PV domain takes it all,
leaving the HVM domain with no slack == crash.

Admittedly, a better solution is to only add on the slack to PV if an
HVM domain is actually running, not merely supported.

BTW, in my testing on 32 bit, a Linux HVM domain consumes an additional
~3 MB over the course of fully booting and starting a few apps.  As I
start more apps, the memory consumption goes up more...

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