[PATCH v15 03/20] arm64: Convert hcalls to use HVC immediate value

[Geoff Levand]

Hi,

On Tue, 2016-03-15 at 13:50 +0000, Dave Martin wrote:
> On Mon, Mar 14, 2016 at 05:48:00PM +0000, Geoff Levand wrote:
> > The existing arm64 hcall implementations are limited in that they only
> > allow for two distinct hcalls; with the x0 register either zero or not
> > zero.  Also, the API of the hyp-stub exception vector routines and the
> > KVM exception vector routines differ; hyp-stub uses a non-zero value in
> > x0 to implement __hyp_set_vectors, whereas KVM uses it to implement
> > kvm_call_hyp.
> > 
> > To allow for additional hcalls to be defined and to make the arm64 hcall
> > API more consistent across exception vector routines, change the hcall
> > implementations to use the 16 bit immediate value of the HVC instruction
> > to specify the hcall type.
> 
> I'm a bit concerned about namespace pollution on the HVC immediate here.
> Existing users tend allocate a single "random" number to identify the
> API -- Xen and Jailhouse do this for example.
> 
> If we start using the HVC immediate to select functions, not just APIs,
> the space is going to fill up a lot faster, if we have a multiplex
> multiple APIs through it.

This was discussed and concluded that we have 16 bits to fill up,
and that is enough.  Functions can still be multiplexed through a
single HVC immediate if the user chooses to do so.

> 
> (We don't currently seem to multiplex APIs much here, except that we
> do use HVC for PSCI calls from the guest, and it could be used for
> additional paravirtualised services in the future).
> 
> > Define three new preprocessor macros HVC_CALL_HYP, HVC_GET_VECTORS, and
> > HVC_SET_VECTORS to be used as hcall type specifiers and convert the
> > existing __hyp_get_vectors(), __hyp_set_vectors() and kvm_call_hyp()
> > routines to use these new macros when executing an HVC call.  Also,
> > change the corresponding hyp-stub and KVM el1_sync exception vector
> > routines to use these new macros.
> 
> It would also be preferable to keep the 32-bit and 64-bit APIs the same;
> we should avoid having them different unless there's a clinching
> technical reason...

Please expand on why you see it as preferable.  What problems do
you see?

-Geoff