On 28 May 14 12:27, Will Deacon wrote:
Hi Larry,
On Mon, May 26, 2014 at 07:56:12PM +0100, Larry Bassel wrote:
To implement the context tracker properly on arm64, a function call needs to be made after debugging and interrupts are turned on, but before the lr is changed to point to ret_to_user(). If the function call is made after the lr is changed the function will not return to the correct place.
For similar reasons, defer the setting of x0 so that it doesn't need to be saved around the function call (save far_el1 in x26 temporarily instead).
Signed-off-by: Larry Bassel larry.bassel@linaro.org
[...]
Why have you added this mov instruction?
I believe (please correct me if I'm wrong) that it is necessary. Here is why:
@@ -476,23 +481,27 @@ el0_undef: // enable interrupts before calling the main handler enable_dbg_and_irq mov x0, sp
- adr lr, ret_to_user b do_undefinstr
el0_dbg: /* * Debug exception handling */ tbnz x24, #0, el0_inv // EL0 only
- mrs x0, far_el1
- mrs x26, far_el1
needed because do_debug_exception may clobber x0, so save far_el1 in x26 (as other parts of this patch do)
- mov x0, x26
needed because far_el1 is expected to be in x0 here
mov x1, x25 mov x2, sp bl do_debug_exception enable_dbg
[call to ct_user_exit will go here in the next patch, this may re-clobber x0]
- mov x0, x26
needed because far_el1 is expected to be in x0 here
Since the purpose of this patch is to make calling a function possible in this code path, the "extra" mov instruction above is necessary and IMHO should be added in this patch and not in the next one whose purpose is to define the ct_user_* macros and add calls to them in the proper places.
b ret_to_user
Will
Larry