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Date: Thu, 29 Nov 2007 18:18:32 +0900
From: "Magnus Damm" <magnus.damm@gmail.com>
Subject: Re: [Qemu-devel] [PATCH]: sh4 delay slot code update
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To: qemu-devel@nongnu.org

On Nov 29, 2007 2:55 PM, Paul Mundt <lethal@linux-sh.org> 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