Re: [PATCH v4 19/28] ARM: KVM: Allow the main HYP code to use the init hyp stub implementation

Christoffer Dall
2017-04-04T07:36:54Z

On Mon, Apr 03, 2017 at 06:51:02PM +0100, Marc Zyngier wrote:
> On 03/04/17 18:32, Christoffer Dall wrote:
> > On Fri, Mar 24, 2017 at 03:01:23PM +0000, Marc Zyngier wrote:
> >> On 24/03/17 14:34, Christoffer Dall wrote:
> >>> On Tue, Mar 21, 2017 at 07:20:49PM +0000, Marc Zyngier wrote:
> >>>> We now have a full hyp-stub implementation in the KVM init code,
> >>>> but the main KVM code only supports HVC_GET_VECTORS, which is not
> >>>> enough.
> >>>>
> >>>> Instead of reinventing the wheel, let's reuse the init implementation
> >>>> by branching to the idmap page when called with a hyp-stub hypercall.
> >>>>
> >>>> Tested-by: Keerthy <j-keerthy@ti.com>
> >>>> Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
> >>>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> >>>> ---
> >>>> arch/arm/kvm/hyp/hyp-entry.S | 29 ++++++++++++++++++++++++-----
> >>>> 1 file changed, 24 insertions(+), 5 deletions(-)
> >>>>
> >>>> diff --git a/arch/arm/kvm/hyp/hyp-entry.S b/arch/arm/kvm/hyp/hyp-entry.S
> >>>> index 1f8db7d21fc5..a35baa81fd23 100644
> >>>> --- a/arch/arm/kvm/hyp/hyp-entry.S
> >>>> +++ b/arch/arm/kvm/hyp/hyp-entry.S
> >>>> @@ -126,11 +126,30 @@ hyp_hvc:
> >>>> */
> >>>> pop {r0, r1, r2}
> >>>>
> >>>> - /* Check for __hyp_get_vectors */
> >>>> - cmp r0, #HVC_GET_VECTORS
> >>>> - mrceq p15, 4, r0, c12, c0, 0 @ get HVBAR
> >>>> - beq 1f
> >>>> + /*
> >>>> + * Check if we have a kernel function, which is guaranteed to be
> >>>> + * bigger than the maximum hyp stub hypercall
> >>>> + */
> >>>> + cmp r0, #HVC_STUB_HCALL_NR
> >>>> + bhs 1f
> >>>>
> >>>> + /*
> >>>> + * Not a kernel function, treat it as a stub hypercall.
> >>>> + * Compute the physical address for __kvm_handle_stub_hvc
> >>>> + * (as the code lives in the idmaped page) and branch there.
> >>>> + * We hijack ip (r12) as a tmp register.
> >>>> + */
> >>>
> >>> How can we just clobber r12 and be sure we don't corrupt the caller?
> >>
> >> r12 (aka ip) is allowed to be clobbered by the linker (used by inserted
> >> code veneers, for example). Given that this is a standalone object, we
> >> can safely assume that r12 has been saved if it was used by the caller.
> >>
> >> Here is what the PCS says:
> >>
> >> "Register r12 (IP) may be used by a linker as a scratch register between
> >> a routine and any subroutine it calls (for details, see
> >> §5.3.1.1, Use of IP by the linker). It can also be used within a routine
> >> to hold intermediate values between subroutine calls."
> >>
> >
> > So isn't this similar to my comment on the arm64 code, which relies on
> > this being called via a function call, as opposed to directly issuring
> > an HVC via inline assembly?
>
> Indeed, this is the exact same thing.
>
> > If so, documenting this limitation/restriction/feature would be nice.
>
> I've added the following to the documentation:
>
> "A stub hypercall is allowed to clobber any of the caller-saved
> registers (x0-x18 on arm64, r0-r3 and ip on arm). It is thus recommended
> to use a function call to perform the hypercall."
>
> Does this work for you?
>
It very much does.

Thanks,
-Christoffer