Re: [RFC PATCH 0/2] QEMU/openbios: PPC Software TLB support in the G4 family

["Cédric Le Goater" <clg@kaod.org>]

>> Right. If we're doing this to say "I can boot a kernel with a 7450 cpu in QEMU" but
>> the implementation is different from real hardware, then I'm not sure what the real
>> value is. That effectively leaves option b) if someone is willing to do the work, or
>> as you say to simply remove the code from QEMU.
> 
> Yeah, that is a good point. Although the software TLB is well contained,
> so we could certainly document that our 7450s don't have that feature
> and call it a day. Does QEMU have any policy on how much of a machine is
> required to be implemented?
> 
> I am more inclined to apply c) for now as I said, just to have some code
> running on the CPU and maybe document in a gitlab issue that we're
> lacking the runtime switch and eventually implement that. It's not like
> this is high traffic code anyway. It has been broken for 10+ years.
> 
> That said, if Cédric and Daniel see more value in moving the 7450s to
> the POWERPC_MMU_32B I won't oppose.

I am in favor of dropping unused code in QEMU and keeping the CPUs for
which we have support in Linux using the POWERPC_MMU_32B in QEMU and the
openbios patch. If we need SoftTLB support for the 74x CPUs in QEMU, we
can always dig in the history.

We can give FreeBSB a try also since they had support for the G4 :

   https://people.freebsd.org/~arved/stuff/minimac


With the openbios patch, Linux boots fine under 7450, 7455, 7447 CPUs.

Under 7448, it drops in xmon with a :
  
kernel tried to execute exec-protected page (c07fdd98) - exploit attempt? (uid: 0)
BUG: Unable to handle kernel instruction fetch
Faulting instruction address: 0xc07fdd98
Vector: 400 (Instruction Access) at [f1019d30]
     pc: c07fdd98: __do_softirq+0x0/0x2f0
     lr: c00516a4: irq_exit+0xbc/0xf8
     sp: f1019df0
    msr: 10001032
   current = 0xc0d00000
     pid   = 1, comm = swapper


This should be fixable.

Thanks,

C.