Re: ia64 kexec: xen -> linux

["Magnus Damm" <magnus.damm@gmail.com>]

On 9/15/06, Horms <horms@verge.net.au> wrote:
> Hi,
>
> as some of you may be aware I am working on porting kexec
> to xen/ia64. I have made a reasoble ammount of progress to this end.
> I'll try and get a new patch set on xen-devel some time next week.
> However I have a problem that I need some ideas on how to solve.
>
> At the moment when kexecing from xen to linux the boot halts on a call
> to efi_gettimeofday(), or more specifically efi.get_time. I'm assuming
> that this is more or less the first efi runtime call that is made, and
> that it is halting because of a discrepancy in the virtual mapping set
> up by efi.set_virtual_address_map().
>
> The problem as I see it is that linux uses a page_offset that covers the
> most significant 3 bits, wherase xen uses the first 4. The unfortunate
> thing is that efi.set_virtual_address_map() can only be called once,
> and I don't think its possible to change the mappings at all once
> its been called.
>
> One idea that I had was to make sure that the efi calls are always made
> in real mode, and never call efi.set_virtual_address_map() at all - efi
> calls have to be made using virtual addressing after
> efi.set_virtual_address_map() is called. But can this work?
>
> Another idea from my colleague Magnus was to map the efi runtime calls
> into some area of memory that is agreed upon by both Linux and Xen (and
> any other kexec-able OS/hypervisor). This seems to be tedious at best.

To clarify this a bit, my plan was to extend the bootloader to provide
some kind resident efi filter code. This code should act as a filter
for all efi run time calls including the dreaded now use-once
set_virtual_address_map() function. The most important task for this
code would be to support an unlimited number of
set_virtual_address_map() calls. I suspect this can be done by always
calling the efi functions from real mode. This technique does however
require switching back and forth to real mode, and I'm not sure how
well that will work out with NMI:s and data that crosses page
boundaries.

A first step would probably be to try to convert Linux into calling
efi runtime functions from real mode. If that works well then the code
can be broken out, made resident and placed into the bootloader.

/ magnus