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.
I plan to fix that after managed to get serial working as that seems to not
need it. If I delete the rtas-size property from /rtas on the original
firmware that makes Linux skip instantiating rtas, but I still get serial
output just not accessing PCI devices. So I think it should work and keeps
things simpler at first. Then I'll try rtas later.
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().
So, we are seeing a problem here because you want the 'sc 1'
interception of vhyp, but not the rest of the stuff that goes with it.
and would using sc 1 for hypercalls on pegasos2 cause other
problems later even if the assert could be removed?
At least in the short term, I think you probably can remove the
assert. In your case the 'sc 1' calls aren't truly to a hypervisor,
but a special case escape to qemu for the firmware emulation. I think
it's unlikely to cause problems later, because nothing on a 32-bit
system should be attempting an 'sc 1'. The only thing I can think of
that would fail is some test case which explicitly verified that 'sc
1' triggered a 0x700 (SIGILL from userspace).
OK so the assert should check if the CPU has an HV bit. I think there was a
#detine for that somewhere that I can add to the assert then I can try
that. What I wasn't sure about is that sc 1 would conflict with the guest's
usage of normal sc calls or are these going through different paths and
only sc 1 will trigger vhyp callback not affecting notmal sc calls? (Or if
this causes an otherwise unnecessary VM exit on KVM even when it works then
maybe looking for a different way in the future might be needed. But for
now if this works with modifying the assert to allow this on ppc32 then I
go for that as that's the simplest way for now.)
Can somebody who knows
more about it explain this please? If this cannot be resolved then we may
need a different hypercall method on pegasos2 (I've considered MOL OSI or
are there other options? I may use some advice from people who know it
better, especially the possible interaction with KVM later as the long
term
goal with pegasos2 is to be able to run with KVM on PPC hardware
eventually.)
Right, you might need an alternative method eventually. Really any
illegal instruction for your cpu is a possible candidate. Bear in
mind that this is *not* truly a hypercall interface, instead it's
something we're special casing for the purposes of faking the
firmware.
The "attn" instruction used on BookE might be a reasonable candidate
(assuming it doesn't conflict with something on 32-bit BookS) - that's
often used for things like signalling the attention of hardware
debuggers, and this is somewhat akin.
Mostly it's just a matter of working out what would be least messy to
intercept in the TCG instruction decoding path.
I'll wait for the current ongoing reorganisations to settle for that. If an
alternative is needed I was considering the interface used by Mac on Linux:
https://lists.nongnu.org/archive/html/qemu-ppc/2021-03/msg00047.html
becuase there are some paravirtual drivers I think that use these on Mac OS
X so this might also be useful for that use case for Mac emulation. But
that seems very similar just checking for magic values at a normal syscall
which means all syscalls will be intercepted anyway. In that case if sc 1
does not interfere with normal sc instructions then it may be better to
keep that as the invalid instruction we trap on.
But this also means that if that assert cannot be dropped or
there may be other problems with sc 1 hypercalls then we maybe cannot
have
the same vof.bin and we'll need a separate version that I would like to
avoid if possible so if there's a simple way to keep it working or make
vof.bin use alternate hypercall method without needing a separate binary
that would be the direction I'd tend to go. Even if we need a seoarate
version I'd like to keep as much common as possible.
I've tested that the missing rtas is not the reason for getting no output
via serial though, as even when disabling rtas on pegasos2.rom it boots
and
I still get serial output just some PCI devices are not detected (such as
USB, the video card and the not emulated ethernet port but these are not
fatal so it might even work as a first try without rtas, just to boot a
Linux kernel for testing it would be enough if I can fix the serial
output).
I still don't know why it's not finding serial but I think it may be some
missing or wrong info in the device tree I generat. I'll try to focus on
this for now and leave the above rtas question for later.
Oh.. another thought on that. You have an ISA serial port on Pegasos,
I believe. I wonder if the PCI->ISA bridge needs some configuration /
initialization that the firmware is expected to do. If so you'll need
to mimic that setup in qemu for the VOF case.
That's what I begin to think because I've added everything to the device
tree that I thought could be needed and I still don't get it working so it
may need some config from the firmware. But how do I access device
registers from board code? I've tried adding a machine reset method and
write to memory mapped device registers but all my attempts failed. I've
tried cpu_stl_le_data and even memory_region_dispatch_write but these did
not get to the device. What's the way to access guest mmio regs from QEMU?