Re: [Xen-devel] Testing nested virtualization on Intel CPUs

[Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>]

On Wed, 21 Sep 2011 17:38:10 +0100
Tim Deegan <tim@xxxxxxx> wrote:

> At 22:18 -0700 on 20 Sep (1316557136), AP wrote:
> > > Yes, booting Xen as a nested guest is very slow at startup, because of
> > > how Xen relocates the bottom 1MB at boot time.  You might find that
> > > 32-bit Xen boots faster.
> > 
> > Could please expand a little on why the relocation causes it to slow down?
> 
> IIRC, the relocation itself is very slow because Xen just copies the low
> 1MB, which includes the VGA hole, so there are a lot of emulated reads.

That seems a bit odd - ISA space reads can have side effects.

Alan

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