Re: [Xen-devel] [PATCH 1/2] arm: implement hypercall continuations

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2012-07-23 at 14:32 +0100, Ian Campbell wrote:
> @@ -482,8 +494,15 @@ static void do_trap_hypercall(struct
> cpu_user_regs *regs, unsigned long iss)
>      regs->r0 = call(regs->r0, regs->r1, regs->r2, regs->r3,
> regs->r4);
>  
>  #ifndef NDEBUG
> -    /* clobber registers */
> -    regs->r1 = regs->r2 = regs->r3 = regs->r4 = regs->r12 =
> 0xDEADBEEF;
> +    /* clobber non-argument registers */
> +    switch ( arm_hypercall_table[regs->r12].nr_args ) {
> +    case 1: regs->r1 = 0xDEADBEEF;
> +    case 2: regs->r2 = 0xDEADBEEF;
> +    case 3: regs->r3 = 0xDEADBEEF;
> +    case 4: regs->r4 = 0xDEADBEEF;
> +            break;
> +    default: BUG();
> +    }
>  #endif
>  } 

This actually inadvertently changes the behaviour to only clobber the
arguments which the particular hypercall does not uses, which is
contrary to the comment in arch-arm.h, which says hypercalls always
clobber all of those registers.

However having spoken to Stefano IRL we think that actually better to
only clobber the ones which are actually used (unless PC has changed, so
continations work). i.e. to change the ABI such that:

diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h
index 6e0fe47..a0593e2 100644
--- a/xen/include/public/arch-arm.h
+++ b/xen/include/public/arch-arm.h
@@ -42,8 +42,9 @@
  *
  * The return value is in r0.
  *
- * The hypercall will always clobber r0, r1, r2, r3, r4 and r12,
- * regardless of how many arguments the particular hypercall takes.
+ * The hypercall will clobber the argument registers, except r0 which
+ * is the return value, i.e. a 2 argument hypercall will clobber r1
+ * and a 4 argument hypercall will clobber r1, r2 and r3.
  *
  */

I'll do this and make the corresponding implementation change and respin
this change on top.


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