Re: [PATCH qemu] spapr: Force 32bit when resetting a core

[Alexey Kardashevskiy]

On 08/01/2022 00:39, Greg Kurz wrote:
> On Fri, 7 Jan 2022 23:19:03 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
>
> > On Fri, Jan 07, 2022 at 12:57:47PM +0100, Greg Kurz wrote:
> > > On Fri, 7 Jan 2022 18:24:23 +1100
> > > Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
> > >
> > > > "PowerPC Processor binding to IEEE 1275" says in
> > > > "8.2.1. Initial Register Values" that the initial state is defined as
> > > > 32bit so do it for both SLOF and VOF.
> > > >
> > > > This should not cause behavioral change as SLOF switches to 64bit very
> > > > early anyway.
> > >
> > > Only one CPU goes through SLOF. What about the other ones, including
> > > hot plugged CPUs ?
> >
> > Those will be started by the start-cpu RTAS call which has its own
> > semantics.
>
> Ah indeed, there's code in linux/arch/powerpc/kernel/head_64.S to switch
> secondaries to 64bit... but then, as noted by Cedric, ppc_cpu_reset(),
> which is called earlier sets MSR_SF but the changelog of commit 8b9f2118ca40
> doesn't provide much details on the motivation. Any idea ?

https://patchwork.kernel.org/project/qemu-devel/patch/1458121432-2855-1-git-send-email-lvivier@redhat.com/

this is probably it:

===
Reset is properly defined as an exception (0x100). For exceptions, the
970MP user manual for example says:

4.5 Exception Definitions
When an exception/interrupt is taken, all bits in the MSR are set to
‘0’, with the following exceptions:
• Exceptions always set MSR[SF] to ‘1’.
===

but it looks like the above is about emulation bare metal 970 rather 
than pseries VCPU so that quote does not apply to spapr.