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Re: [Xen-devel] [PATCH v3 27/32] libxc: change the position of the special pages

On Fri, Jul 03, 2015 at 05:36:15PM +0200, Roger Pau Monné wrote:
> 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.

Could stick at the start of memory, mark it as E820_RESV or such. Or
at 16MB or such?
> Roger.
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