[PATCH 3/3] xen: arm: handle thumb mode guest in continuations

[PATCH 3/3] xen: arm: handle thumb mode guest in continuations

[Ian Campbell]

PC only needs adjusting by 2, otherwise we rerun the instruction prior
to the hvc as well.

hvc is unpredictable if used within a Thumb IT (conditional execution)
block, so we don't need to worry about rewinding any of that state.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
---
Should be backported
---
 xen/arch/arm/domain.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index 10f13e4..2d2197e 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -387,10 +387,11 @@ unsigned long hypercall_create_continuation(
         else
 #endif
         {
+            int is_thumb = (regs->cpsr & PSR_THUMB);
             regs->r12 = op;
 
             /* Ensure the hypercall trap instruction is re-executed. */
-            regs->pc -= 4;  /* re-execute 'hvc #XEN_HYPERCALL_TAG' */
+            regs->pc -= is_thumb?2:4; /* re-execute 'hvc #XEN_HYPERCALL_TAG' */
 
             for ( i = 0; *p != '\0'; i++ )
             {
-- 
1.7.10.4

Re: [PATCH 3/3] xen: arm: handle thumb mode guest in continuations

[Julien Grall]

Hi Ian,

On 25/03/2015 15:34, Ian Campbell wrote:
> PC only needs adjusting by 2, otherwise we rerun the instruction prior
> to the hvc as well.

I don't understand why you have to adjust PC by 2 for thumb.
The spec encodes the HVC thumb instruction on 32 bits (i.e 4 bytes).

Regards,


-- 
Julien Grall

Re: [PATCH 3/3] xen: arm: handle thumb mode guest in continuations

[Ian Campbell]

On Wed, 2015-03-25 at 23:33 +0000, Julien Grall wrote:
> Hi Ian,
> 
> On 25/03/2015 15:34, Ian Campbell wrote:
> > PC only needs adjusting by 2, otherwise we rerun the instruction prior
> > to the hvc as well.
> 
> I don't understand why you have to adjust PC by 2 for thumb.
> The spec encodes the HVC thumb instruction on 32 bits (i.e 4 bytes).

I was confused by Encoding T1 in the ARMv7 ARM B9.3.4 which has a 15 at
the left most column, but actually it is 15..0,15..0, doh!

So we are fine here. I'll drop this patch and pull the regs->pc frobbing
in the previous patch back out of the if into the common bit.

Ian.