Re: [PATCH qemu v20] spapr: Implement Open Firmware client interface

[David Gibson]

On Mon, May 24, 2021 at 02:42:30PM +0200, BALATON Zoltan wrote:
> On Mon, 24 May 2021, David Gibson wrote:
> > On Sun, May 23, 2021 at 07:09:26PM +0200, BALATON Zoltan wrote:
> > > On Sun, 23 May 2021, BALATON Zoltan wrote:
> > > > On Sun, 23 May 2021, Alexey Kardashevskiy wrote:
> > > > > One thing to note about PCI is that normally I think the client
> > > > > expects the firmware to do PCI probing and SLOF does it. But VOF
> > > > > does not and Linux scans PCI bus(es) itself. Might be a problem for
> > > > > you kernel.
> > > > 
> > > > I'm not sure what info does MorphOS get from the device tree and what it
> > > > probes itself but I think it may at least need device ids and info about
> > > > the PCI bus to be able to access the config regs, after that it should
> > > > set the devices up hopefully. I could add these from the board code to
> > > > device tree so VOF does not need to do anything about it. However I'm
> > > > not getting to that point yet because it crashes on something that it's
> > > > missing and couldn't yet find out what is that.
> > > > 
> > > > I'd like to get Linux working now as that would be enough to test this
> > > > and then if for MorphOS we still need a ROM it's not a problem if at
> > > > least we can boot Linux without the original firmware. But I can't make
> > > > Linux open a serial console and I don't know what it needs for that. Do
> > > > you happen to know? I've looked at the sources in Linux/arch/powerpc but
> > > > not sure how it would find and open a serial port on pegasos2. It seems
> > > > to work with the board firmware and now I can get it to boot with VOF
> > > > but then it does not open serial so it probably needs something in the
> > > > device tree or expects the firmware to set something up that we should
> > > > add in pegasos2.c when using VOF.
> > > 
> > > I've now found that Linux uses rtas methods read-pci-config and
> > > write-pci-config for PCI access on pegasos2 so this means that we'll
> > > probably need rtas too (I hoped we could get away without it if it were 
> > > only
> > > used for shutdown/reboot or so but seems Linux needs it for PCI as well 
> > > and
> > > does not scan the bus and won't find some devices without it).
> > 
> > Yes, definitely sounds like you'll need an RTAS implementation.
> > 
> > > While VOF can do rtas, this causes a problem with the hypercall method 
> > > using
> > > sc 1 that goes through vhyp but trips the assert in ppc_store_sdr1() so
> > > cannot work after guest is past quiesce.
> > 
> > > So the question is why is that
> > > assert there
> > 
> > Ah.. right.  So, vhyp was designed for the PAPR use case, where we
> > want to model the CPU when it's in supervisor and user mode, but not
> > when it's in hypervisor mode.  We want qemu to mimic the behaviour of
> > the hypervisor, rather than attempting to actually execute hypervisor
> > code in the virtual CPU.
> > 
> > On systems that have a hypervisor mode, SDR1 is hypervisor privileged,
> > so it makes no sense for the guest to attempt to set it.  That should
> > be caught by the general SPR code and turned into a 0x700, hence the
> > assert() if we somehow reach ppc_store_sdr1().
> 
> This seems to work to avoid my problem so I can leave vhyp enabled after
> qiuesce for now:
> 
> diff --git a/target/ppc/cpu.c b/target/ppc/cpu.c
> index d957d1a687..13b87b9b36 100644
> --- a/target/ppc/cpu.c
> +++ b/target/ppc/cpu.c
> @@ -70,7 +70,7 @@ void ppc_store_sdr1(CPUPPCState *env, target_ulong value)
>  {
>      PowerPCCPU *cpu = env_archcpu(env);
>      qemu_log_mask(CPU_LOG_MMU, "%s: " TARGET_FMT_lx "\n", __func__, value);
> -    assert(!cpu->vhyp);
> +    assert(!cpu->env.has_hv_mode || !cpu->vhyp);
>  #if defined(TARGET_PPC64)
>      if (mmu_is_64bit(env->mmu_model)) {
>          target_ulong sdr_mask = SDR_64_HTABORG | SDR_64_HTABSIZE;
> 
> But I wonder if the assert should also be moved within the TARGET_PPC64
> block and if we may need to generate some exception here instead. Not sure
> what a real CPU would do in this case but if accessing sdr1 is privileged in
> HV mode then there should be an exception or if that's catched
> elsewhere

It should be caught elsehwere.  Specifically, when the SDR1 SPR is
registered, on CPUs with a hypervisor mode it should be registered as
hypervisor privileged, so the general mtspr dispatch logic should
generate the exception if it's called from !HV code.  The assert here
is just to sanity check that it has done so before we enter the actual
softmmu code.

> then this assert may not be needed at all. I can make a patch if you tell me
> what should it do.
> 
> Regards,
> BALATON Zoltan
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

signature.asc
Description: PGP signature