[RFC][PATCH] powerpc/64s: optimise syscall entry with relon hypercalls

[David Laight]

From: Nicholas Piggin
> Sent: 10 February 2017 18:23
> After bc3551257a ("powerpc/64: Allow for relocation-on interrupts from
> guest to host"), a getppid() system call goes from 307 cycles to 358
> cycles (+17%). This is due significantly to the scratch SPR used by the
> hypercall.
> 
> It turns out there are a some volatile registers common to both system
> call and hypercall (in particular, r12, cr0, ctr), which can be used to
> avoid the SPR and some other overheads for the system call case. This
> brings getppid to 320 cycles (+4%).
...
> + * syscall register convention is in Documentation/powerpc/syscall64-abi.txt
> + *
> + * For hypercalls, the register convention is as follows:
> + * r0 volatile
> + * r1-2 nonvolatile
> + * r3 volatile parameter and return value for status
> + * r4-r10 volatile input and output value
> + * r11 volatile hypercall number and output value
> + * r12 volatile
> + * r13-r31 nonvolatile
> + * LR nonvolatile
> + * CTR volatile
> + * XER volatile
> + * CR0-1 CR5-7 volatile
> + * CR2-4 nonvolatile
> + * Other registers nonvolatile
> + *
> + * The intersection of volatile registers that don't contain possible
> + * inputs is: r12, cr0, xer, ctr. We may use these as scratch regs
> + * upon entry without saving.

Except that they must surely be set to some known value on exit in order
to avoid leaking information to the guest.

	David