[PATCH v4 07/28] arm64: KVM: Allow the main HYP code to use the init hyp stub implementation

On Fri, Mar 24, 2017 at 02:56:51PM +0000, Marc Zyngier wrote:

On 24/03/17 14:33, Christoffer Dall wrote:

quoted

On Tue, Mar 21, 2017 at 07:20:37PM +0000, Marc Zyngier wrote:

quoted

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.

Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <redacted>
---
 arch/arm64/kvm/hyp/hyp-entry.S | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
index d8ef788646c6..4f34c5996f86 100644
--- a/arch/arm64/kvm/hyp/hyp-entry.S
+++ b/arch/arm64/kvm/hyp/hyp-entry.S

@@ -87,10 +87,24 @@ alternative_endif
 	/* Here, we're pretty sure the host called HVC. */
 	ldp	x0, x1, [sp], #16
 
-	cmp	x0, #HVC_GET_VECTORS
-	b.ne	1f
-	mrs	x0, vbar_el2
-	b	2f
+	/* Check for a stub HVC call */
+	cmp	x0, #HVC_STUB_HCALL_NR
+	b.hs	1f
+
+	/*
+	 * Compute the idmap address of __kvm_handle_stub_hvc and
+	 * jump there. Since we use kimage_voffset, do not use the
+	 * HYP VA for __kvm_handle_stub_hvc, but the kernel VA instead
+	 * (by loading it from the constant pool).
+	 *
+	 * Preserve x0-x4, which may contain stub parameters.
+	 */
+	ldr	x5, =__kvm_handle_stub_hvc
+	ldr_l	x6, kimage_voffset

Isn't it a bit dodgy to just overwrite x5 and x6 in something which is
not a function?  I know that in practice this always gets called through
a function call and we can rely on the calling convention, but this can
break if you issue a hypercall to KVM's HVC sub implementation using
inline assembly, I think.

Am I missing something here?

I don't think you're missing anything. We're definitely relying on
getting there via a function call which is going to preserve the
caller-saved registers.

The only case where we issue a naked HVC call is when we use
HVC_SOFT_RESTART. At this point, corrupted registers is not much of a worry.

Now, I see two ways of dealing with this:
- We save x5/x6 on the stack, restoring them in the stub handler , right
before the eret
- Or we simply document the behaviour in asm/virt.h so that people
adding new stub methods know the restrictions.

Thoughts?

I'm perfectly happy with documenting the feature and documenting it
here.  It just felt arbitrary to me when I read the code that we could
just step on x5/x6.

Thanks,
-Christoffer