RE: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on

[David Laight <David.Laight@ACULAB.COM>]

From: Will Deacon
> Sent: 29 January 2018 13:20
...
> Another issue with this style of macro definition exists on architectures
> where the calling convention needs you to carry state around depending on
> how you packed the previous parameters. For example, on 32-bit ARM, 64-bit
> values are passed in adjacent pairs of registers but the low numbered
> register needs to be even. This is what stopped me from trying to use
> existing helpers such as syscall_get_arguments to unpack the pt_regs
> and it generally means that anything that says "get me argument n" is going
> to require constructing arguments 0..n-1 first.
> 
> To do this properly I think we'll either need to pass back the size and
> current register offset to the arch code, or just allow the thing to be
> overridden per syscall (the case above isn't especially frequent).

If you generate a structure from the argument list that might work
'by magic'.
Certainly you can add explicit pads to any structure.

	David