Re: [Qemu-devel] [PATCH] MIPS interrupts and -icount

[Aurelien Jarno <aurelien@aurel32.net>]

On Sun, Jul 25, 2010 at 07:46:49AM +0200, Edgar E. Iglesias wrote:
> On Sun, Jul 25, 2010 at 05:08:07AM +0200, Aurelien Jarno wrote:
> > On Sun, Jul 25, 2010 at 02:07:54AM +0200, Edgar E. Iglesias wrote:
> > > On Sun, Jul 25, 2010 at 12:55:45AM +0200, Aurelien Jarno wrote:
> > > > On Thu, Jul 22, 2010 at 01:32:18PM +0200, Edgar E. Iglesias wrote:
> > > > > Hi,
> > > > > 
> > > > > I'm seeing an error when emulating MIPS guests with -icount.
> > > > > In cpu_interrupt:
> > > > >   cpu_abort(env, "Raised interrupt while not in I/O function");
> > > > 
> > > > I am not able to reproduce the issue. Do you have more details how to
> > > > reproduce the problem?
> > > 
> > > You need a machine with devices that raise the hw interrupts. I didn't
> > > see the error on the images on the wiki though. But I've got a machine
> > > here that trigs it easily. Will check if I can publish it and an image.
> > > 
> > 
> > That would be nice if you can share it.
> > 
> > > > > It seems to me like the MIPS interrupt glue logic between interrupt
> > > > > controllers and the MIPS core is not modeled correctly.
> > > > 
> > > > It seems indeed that sometimes interrupt are triggered while not in 
> > > > I/O functions, your patch addresses part of the problem.
> > > > 
> > > > > When hw interrupt pending bits in CP0_Cause are set, the CPU should
> > > > > see the hw interrupt line as active. The CPU may or may not take the
> > > > > interrupt based on internal state (global irq mask etc) but the glue
> > > > > logic shouldn't care about that. Am I missing something here?
> > > > 
> > > > I don't think it is correct. On the real hardware, the interrupt line is
> > > > actually active only when all conditions are fulfilled.
> > > > 
> > > > The thing to remember is that the interrupts are level triggered. So if
> > > > an interrupt is masked, it should be rejected by the CPU, but could be
> > > > triggered again as soon as the interrupt mask is changed.
> > > 
> > > Agreed, that behaviour is what I'm trying to acheive. The problem now
> > > is that the the level triggered line, prior to CPU masking is beeing masked
> > > by external logic. IMO it shouldnt. See hw/mips_int.c and cpu-exec.c prior
> > > to the patch.
> > 
> > Actually all depends if you consider the MIPS interrupt controller part
> > of the CPU or not. It could be entirely modeled in the CPU, that is in 
> > cpu-exec.c or entirely modeled as a separate controller, that is in 
> > mips_int.c.
> > 
> > IMHO it should be in mips_int.c. It is an interrupt controller like
> > another that combines a few interrupt lines into a single one that feeds
> > the CPU. It is like for example the i8259, with the exception that the 
> > configuration is not done by load/store into MMIO area, but directly 
> > using CPU special registers. We should probably mark these instructions 
> > as I/O.
> 
> 
> Hi,
> 
> I agree that it's not obvious where things should be modeled, I'll try to
> explain my view.
> 
> As a first step I'm trying to model a MIPS configured with Vectored
> Interrupts. We've got external interrupt logic feeding the hw
> interrupt lines. These lines are level triggered, held active by
> the external logic as long as interrupts are pending. Regardless
> of wether the CPU want's to take the interrupt now or later. In fact,
> there is no way to access the internal flags from RTL logic located
> here (AFAIK). In my mind, this layers pretty much ends in hw/mips_int.c.
> 
> Internally in the MIPS core, I'm guessing there is logic that simpliy
> applies the internal CPU masks, outputing a single internal IRQ line
> that decides wether the CPU should take the IRQ or not. Here, things like
> IE flags etc matter. I don't have access to RTL on the MIPS side so I'm
> just guessing here.
> 
> In my mind, we should model this latter part by asserting INTERRUPT_HARD
> from hw/mips_int.c whenever any hw lines are active and letting the
> CPU in cpu-exec.c decide when to take the interrupt by applying it's
> internal masking.
> 

Sorry to come back so long after this discussion, but I now have another
argument. This commit causes a regression, the host CPU is now always at
100%. QEMU spent all its time looping because the CPU interrupt line is
asserted.

Not asserting the CPU interrupt line when interrupts are disabled fixes
the issue.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net