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

From: [email-masked] (Christoffer Dall)
Date: Mon, 3 Apr 2017 19:32:25 +0200

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 <[email-masked]>
> >> Acked-by: Russell King <[email-masked]>
> >> Signed-off-by: Marc Zyngier <[email-masked]>
> >> ---
> >>  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?

If so, documenting this limitation/restriction/feature would be nice.

Thanks,
-Christoffer