Re: [PATCH 1/4] Documentation: arm: [U]EFI runtime services

[James Bottomley <James.Bottomley-d9PhHud1JfjCXq6kfMZ53/egYHeGw8Jk@public.gmane.org>]

On Thu, 2013-06-27 at 07:23 +0100, Grant Likely wrote:
> On Thu, Jun 27, 2013 at 2:32 AM, Matthew Garrett <mjg59-1xO5oi07KQx4cg9Nei1l7Q@public.gmane.org> wrote:
> > On Wed, Jun 26, 2013 at 07:38:19AM -0700, James Bottomley wrote:
> >> The fixed virtual address scheme currently being looked at for x86_64 to
> >> make SetVirtualAddressMap() kexec invariant doesn't work on 32 bit
> >> because the address space isn't big enough.  For ARM, given that we've
> >> much more opportunity to work with the vendors, can we just avoid
> >> transitioning to a virtual address map and always just install a
> >> physical mapping before doing efi calls?
> >
> > We can probably get away with that now, but it does risk us ending up
> > with some firmware that expects to run in physical mode (boards designed
> > for Linux) and some firmware that expects to run in virtual mode (boards
> > designed for Windows). The degree of lockdown in the Windows ecosystem
> > at present means it's not a real problem at the moment, but if that ever
> > changes we're going to risk incompatibility.
> 
> What is the problem trying to be avoided by not using the virtual map?
> Is it passing the virtual mapping data from one kernel to the next
> when kexecing? Or something else?

Where to begin ... SetVirtualAddressMap() is one massive hack job ...
just look at the tiano core implementation.   Basically it has a fixed
idea of where all the pointers are and it tries to convert them all to
the new address space.  The problem we see in x86 is that this
conversion process isn't exhaustive due to implementation cockups, so
the post virtual address map image occasionally tries to access
unconverted pointers via the old physical address and oopses the kernel.

The problem for kexec is that SetVirtualAddressMap isn't idempotent.  In
fact by API fiat it can only ever be called once for the entire lifetime
of the UEFI bios, which could be many kernels in a kexec situation.  So,
somehow the subsequent kernels have to know not to call it, plus,
obviously, the virtual address map of the previous kernel has to work in
the next because it can't set up a new one.

James