[RESEND PATCH v4 8/8] arm64: Allow 64-bit tasks to invoke compat syscalls

[Ryan Houdek]

On Tue, May 18, 2021 at 1:26 PM David Laight <David.Laight at aculab.com> wrote:
>
> From: Arnd Bergmann
> > Sent: 18 May 2021 14:02
> ...
> >
> > I'm still undecided about this approach. It is an easy way to expose the 32-bit
> > ABIs, it mostly copies what x86-64 already does with 32-bit syscalls and
> > it doesn't expose a lot of attack surface that isn't already exposed to normal
> > 32-bit tasks running compat mode.
> >
> > On the other hand, exposing the entire aarch32 syscall set seems both
> > too broad and not broad enough: Half of the system calls behave the
> > exact same way in native and compat mode, so they wouldn't need to
> > be exposed like this, a lot of others are trivially emulated in user space
> > by calling the native versions. The syscalls that are actually hard to do
> > such as ioctl() or the signal handling will work for aarch32 emulation, but
> > they are still insufficient to correctly emulate other 32-bit architectures
> > that have a slightly different ABI. This means the interface is a fairly good
> > fit for Tango, but much less so for FEX.
>
> To my mind because the kernel already contains the emulation code
> there isn't much point trying to replicate it in userspace.
>
> OTOH I think they are trying to emulate x86 system calls not arm ones.
> So the structure layouts don't always match.
> However it is probably a lot nearer than the 64bit arm.

Take care not to conflate the Tango and FEX project needs here.
Tango is doing aarch32->aarch64 translation. So they are translating
aarch32 syscalls.
FEX is doing {x86, x86-64}->aarch64 translation.
The simplicity of the interface helps Tango more than FEX in this regard.
Since FEX likely still needs userspace fixups to *some* structures.

>
> Whether including some of the 'x32' code in an arm kernel will
> help is another matter - it might be a useful source of differences.
>
> Am I also right in thinking that this isn't actually needed as part
> of a 'generic' ARM kernel? Just ones for some specific platforms?

This isn't correct from FEX's viewpoint.
FEX isn't a product that will be shipping on any specific platform; The user
is expected to just install FEX on their ARMv8.0+ device of choice.
After they install FEX then they will freely be able to install *any* x86/x86-64
software and run it.
The primary target is running full fledged games from the user's Steam library,
but it can be anything the user desires.

For context we have users running FEX on Lenovo c630, Apple M1, HardKernel SBCs,
and recently some randomly picked Android devices.

I can't speak on Tango's behalf here since I don't know their user ecosystem.
>
>         David
>
> (Oh - I'm not involved in the project and will probably never use it.)
>
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