Re: [Qemu-devel] [PULL 07/17] ppc: Fix rfi/rfid/hrfi/... emulation

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2016-06-23 at 15:48 +1000, David Gibson wrote:
> From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> 
> This reworks emulation of the various "rfi" variants. I removed
> some masking bits that I couldn't make sense of, the only bit that
> I am aware we should mask here is POW, the CPU's MSR mask should
> take care of the rest.

See I'd rather we didn't boot at all.

I just spent hours trying to figure out why my kernel wouldn't boot
in qemu on a mac99 model with 970, weird weird things happening
inside the device-tree parsing...

Until I figured we were losing the 64-bit mode in the MSR. Why ?

Because OpenBIOS isn't bolting the hash entries or SLBs for the entire
kernel ! So we are taking some exceptions right during the early
assembly, precisely between enable_64b_mode and __mmu_off.

Now this is really fishy to begin with, there is code in there that
will use SRR0/SRR1 and won't expect a fault of any sort... such
as __mmu_off itself.

The problem in our case was that OpenBIOS using rfi, it only restores
32-bits of the MSR, so we lose the 64-bit flag.

Typically that was happening on the call to __cpu_preinit_ppc970 which
happens to reside far enough away that it needs a new translation.

I wonder if prom_init should "touch" the entire kernel for safety,
but in any case, OpenBIOS need that fix urgently.

Cheers,
Ben.