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From: =?UTF-8?B?Um9nZXIgUGF1IE1vbm7DqQ==?= <roger.pau@citrix.com>
Subject: Re: [PATCH v3 27/32] libxc: change the position of the
	special pages
Date: Fri, 3 Jul 2015 17:36:15 +0200
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Cc: Wei Liu <wei.liu2@citrix.com>, Ian Jackson <ian.jackson@eu.citrix.com>, Ian Campbell <ian.campbell@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>
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El 03/07/15 a les 13.35, Roger Pau Monne ha escrit:
> Change the physical memory address of the special pages when there are no
> emulated devices. On HVM guests the special pages have always been reserved
> so that they end at the 0xff000 pfn, but there are some problems with this
> approach when used without emulated devices:
> 
>  - If we want to allow HVMlite to be used for Dom0 those special pages
>    cannot be placed inside of the MMIO hole or else they may clash with MMIO
>    regions of physical devices passed-through to Dom0.
>  - If there's no emulated devices the guests needs to access the console
>    page and the command line very early during boot. This is a problem for
>    FreeBSD at least, since the early page tables only map memory up to 1GiB.
> 
> So instead append the special pages after the kernel and ramdisk. The guest
> must make sure it figures out the position of those pages very early during
> boot, since the region is not marked as reserved in the memory map.

After thinking a little bit about this, I don't think this is the right
solution. Placing all the special pages after the kernel/ramdisk will
make it quite hard to use bootloaders like pvgrub inside of HVMlite
guests. I will see if I can come up with a better way to solve this,
comments welcome.

Roger.