Hi Paul, Thanks for your comments.
On Nov 28, 2007 9:49 PM, Paul Mundt <[EMAIL PROTECTED]> wrote: > On Wed, Nov 28, 2007 at 06:54:20PM +0900, Magnus Damm wrote: > > +#define DELAY_SLOT_TRUE (1 << 2) > > +#define DELAY_SLOT_CLEARME (1 << 3) > > +/* The dynamic value of the DELAY_SLOT_TRUE flag determines whether the > > jump > > + * after the delay slot should be taken or not. It is calculated from SR_T. > > + * > > + * It is unclear if it is permitted to modify the SR_T flag in a delay > > slot. > > + * The use of DELAY_SLOT_TRUE flag makes us accept such SR_T modification. > > + */ > > Nesting a 'tst' in a delay slot is certainly valid, and GCC correctly > treats it as a slottable instruction. If you're in doubt as to whether an > opcode can be placed in a delay slot or not, the machine descriptor is a > good way of sorting things out. The only restrictions I know of things > that cause changes to PC, most of the system instructions (like trapa and > ldtlb), and so on. There are of course cases where an instruction itself > is slottable which may perform illegal behaviour via PC modification or > so on, and we do have an exception for trapping that sort of abuse. I was mainly wondering if I really needed to save the state of SR_T, but I assumed so. So the code should be correct. And yes, I'm sure there are quite a few slottable instructions with interesting side effects, but that's a separate issue. > You can see an example in arch/sh/kernel/entry-common.S: > > syscall_exit_work: > ! r0: current_thread_info->flags > ! r8: current_thread_info > tst #_TIF_SYSCALL_TRACE | _TIF_SINGLESTEP | > _TIF_SYSCALL_AUDIT, r0 > bt/s work_pending > tst #_TIF_NEED_RESCHED, r0 > > .... > work_pending: > ! r0: current_thread_info->flags > ! r8: current_thread_info > ! t: result of "tst #_TIF_NEED_RESCHED, r0" > bf/s work_resched > tst #(_TIF_SIGPENDING | _TIF_RESTORE_SIGMASK), r0 > > .... > > This sort of access is not a particularly rare workload. Presumably you'd hit > this under system emulation at the very least. Yeah, that's a pretty good example that shows that I need to save the SR_T state before executing the delay slot instruction. Thanks for pointing out that code. / magnus
