[RFC PATCH] arm64: use non-global mappings for UEFI runtime regions

[will.deacon@arm.com (Will Deacon)]

On Tue, Nov 17, 2015 at 06:00:36PM +0100, Ard Biesheuvel wrote:
> On 17 November 2015 at 17:48, Mark Rutland <mark.rutland@arm.com> wrote:
> > On Tue, Nov 17, 2015 at 04:34:46PM +0000, Will Deacon wrote:
> >> On Tue, Nov 17, 2015 at 03:25:58PM +0000, Mark Rutland wrote:
> >> > Will, are you aware of anything that could bite us here?
> >>
> >> Can we guarantee that efi_virtmap_{load,unload} are called with interrupts
> >> enabled?
> >
> > Unfortuantely, it looks like we can guarantee interrupts are _disabled_.
> >
> > Every function in drivers/firmware/efi/runtime-wrappers.c which uses
> > efi_call_virt (and hence efi_virtmap_{load,unload}) wraps the call in a
> > spin_lock_irq{save,restore} pair. Those appear to be the only uses of
> > efi_call_virt.
> >
> 
> There is actually no need from the UEFI pov to invoke the UEFI runtime
> services with interrupts disabled, this is simply an implementation
> detail of the kernel support, and I think it is primarily for x86 (but
> I have to dig up the old thread for the details)

So you have a double-edged sword here:

  - switch_mm must be called with interrupts enabled prior to -rc1,
    otherwise we play a song-and-dance with TIF_SWITCH_MM.

  - If you have interrupts enabled, you can receive a rollover IPI from
    another core, which means you switch to current->active_mm.

Will