Android and compatibility with deprecated armv7 instructions

[arnd@arndb.de (Arnd Bergmann)]

On Saturday 05 July 2014, Catalin Marinas wrote:
> On 5 Jul 2014, at 19:43, Arnd Bergmann <arnd@arndb.de> wrote:
>
> > I think if a there is no way to turn off a feature, we can't really
> > treat it as "deprecated", since we are lacking the most important tool
> > to help users migrate away from it. Do you know why the architecture
> > folks believe it makes sense to have distinct steps 1 and 2?
> 
> During this phase you usually only get compiler warnings which are
> clearly not enough for already built apps.
> 
> But it looks like this is changing with ARMv8. SETEND is deprecated and
> disable bit available. IT instruction also deprecated with a disable bit
> (basically only allowing for one subsequent conditional instruction,
> though I don?t think we can easily get rid of this one in user space).

Ok. I can also see ways to emulate SETEND from kernel side by modifying
the user pt_regs, but I don't see how we could emulate IT without anything
short of a full interpretation of user instructions from the kernel.

> > Another problem that I see with the way that features are phased out
> > on ARM is how the hypervisor architecture makes it really hard to
> > run a guest in an older architecture version. For instance on s390,
> > you can basically emulate any prior machine from the past 50 years
> > by selectively trapping some of the instructions from the guest
> > into the hypervisor, at least for any instruction that has had
> > a different behavior in the past.
> 
> That?s indeed not possible (basically a SWP at EL1 would trap as undef
> at EL1 rather than EL2). But is there much value in this? Do we have a
> large base of pre-built OS kernels? We are trying hard to get to single
> Image, let alone using old builds of a kernel.

There are countless reasons why you'd want this actually, including:

- running an old arm7tdmi rtos build that you lost the source code for
  but that would be cheaper to run on a new cortex-a7 emulating the
  peripherals than to rewrite and revalidate

- running OABI binaries in a 32-bit guest on an armv8 (or future version)

- testing armv4 kernel builds in a kvm guest using qemu models

- running Windows CE binaries in a virtual machine

- running an ARMv8 SBSA based OS on ARMv9 hardware

- ...

The single image approach can by definition only address the hardware
we already know, while one of the main uses of virtualization is to
run existing software on hardware that was unknown at the time the
binaries were compiled.

	Arnd