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Re: [Qemu-devel] [PATCH]: sh4 delay slot code update
From: |
Magnus Damm |
Subject: |
Re: [Qemu-devel] [PATCH]: sh4 delay slot code update |
Date: |
Thu, 29 Nov 2007 18:18:32 +0900 |
On Nov 29, 2007 2:55 PM, Paul Mundt <address@hidden> wrote:
> The only thing to be careful of is the ordering semantics for the T-bit
> state, which is what I tried to convey in the code snippet.
>
> > > 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
> > >
> So while bt/s is conditional on the first flag test, the T-state is
> relative to the second test by the time the branch happens.
Exactly.
> The T-state check for bt/bf happens _prior_ to execution of the delay
> slot instruction, while any delay-slot resident T-bit modifier is
> executed by the time we enter the branch. I don't know if your code
> handles that or not, but figured it's probably good to make that
> explicit. T-bit modifiers are always a bit hairy..
Yeah, the T flag is indeed a bit hairy. The patch should handle the T
bit exactly as you describe it. As part of the bt/bf instruction the T
flag is evaluated and if true the the DELAY_SLOT_TRUE bit is set. This
bit is later used to decide if we should jump or not after the delay
slot instruction.
Thank you,
/ magnus