[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

From: BALATON Zoltan
Subject: Re: [PATCH qemu v20] spapr: Implement Open Firmware client interface
Date: Fri, 4 Jun 2021 15:59:22 +0200 (CEST)

On Fri, 4 Jun 2021, David Gibson wrote:
On Wed, Jun 02, 2021 at 02:29:29PM +0200, BALATON Zoltan wrote:
On Wed, 2 Jun 2021, David Gibson wrote:
On Thu, May 27, 2021 at 02:42:39PM +0200, BALATON Zoltan wrote:
On Thu, 27 May 2021, David Gibson wrote:
On Tue, May 25, 2021 at 12:08:45PM +0200, BALATON Zoltan wrote:
On Tue, 25 May 2021, David Gibson wrote:
On Mon, May 24, 2021 at 12:55:07PM +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.

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?

The vhyp shouldn't affect normal system calls, 'sc 1' is specifically
for hypercalls, as opposed to normal 'sc' (a.k.a. 'sc 0'), and the
vhyp only intercepts the hypercall version (after all Linux on PAPR
certainly uses its own system calls, and hypercalls are active for the
lifetime of the guest there).

(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.

What you're doing here won't work with KVM as it stands.  There are
basically two paths into the vhyp hypercall path: 1) from TCG, if we
interpret an 'sc 1' instruction we enter vhyp, 2) from KVM, if we get
a KVM_EXIT_PAPR_HCALL KVM exit then we also go to the vhyp path.

The second path is specific to the PAPR (ppc64) implementation of KVM,
and will not work for a non-PAPR platform without substantial
modification of the KVM code.

OK so then at that point when we try KVM we'll need to look at alternative
ways, I think MOL OSI worked with KVM at least in MOL but will probably make
all syscalls exit KVM but since we'll probably need to use KVM PR it will
exit anyway. For now I keep this vhyp as it does not run with KVM for other
reasons yet so that's another area to clean up so as a proof of concept
first version of using VOF vhyp will do.

Eh, since you'll need to modify KVM anyway, it probably makes just as
much sense to modify it to catch the 'sc 1' as MoL's magic thingy.

I'm not sure how KVM works for this case so I also don't know why and what
would need to be modified. I think we'll only have KVM PR working as newer
POWER CPUs having HV (besides being rare among potential users) are probably
too different to run the OSes that expect at most a G4 on pegasos2 so likely
it won't work with KVM HV.

Oh, it definitely won't work with KVM HV.

If we have KVM PR doesn't sc already trap so we
could add MOL OSI without further modification to KVM itself only needing
change in QEMU?

Uh... I guess so?

I also hope that MOL OSI could be useful for porting some
paravirt drivers from MOL for running Mac OS X on Mac emulation but I don't
know about that for sure so I'm open to any other solution too.

Maybe.  I never know much about MOL to begin with, and anything I did
know was a decade or more ago so I've probably forgotten.

That may still be more than what I know about it since I never had any
knowledge about PPC KVM and don't have any PPC hardware to test with so I'm
mostly guessing. (I could test with KVM emulated in QEMU and I did set up an
environment for that but that's a bit slow and inconvenient so I'd leave KVM
support to those interested and have more knowledge and hardware for it.)

Sounds like a problem for someone else another time, then.

For now I'm
going with vhyp which is enough fot testing with TCG and if somebody wants
KVM they could use he original firmware for now so this could be improved in
a later version unless a simple solution is found before the freeze for 6.1.
If we're in KVM PR what happens for sc 1 could that be used too so maybe
what we have now could work?

Note that if you do go down the MOL path it wouldn't be that complex
to make a "vMOL" interface so you can use the same mechanism for KVM
and TCG.

Not sure what you mean by VMOL. Is it modifying MOL to use sc 1 like VOF
instead of its OSI way for hypercalls?

No, I mean on the qemu side adding an optional hook which will
intercept sc 0 instructions with the MOL magic register values and
redirect them to a machine registered callback, rather than emulating
the CPU's behaviour of jumping to the system call vector in guest

Basically an equivalent of vhyp, but for MOL magic syscalls, instead
of hypercalls.

OK, that's basically what BenH's OSI patch I've linked to before did I think, it may just need updating for changes in target/ppc since that patch was created. However that would also mean we'd need another version of VOF that uses this instead of sc 1 then so unless we need that I'd keep a single VOF that works for both spapr and pegasos2.

That would lose the advantage of
being able to reuse MOL guest drivers without modification (which might be
useful for running OS X guest on Mac emulation) so if we can't use vhyp then
maybe using OSI would be the next choice for this reason but for now vhyp
seems to be working for what I could test so unless somebody here sees a
problem with it and has a better idea I'm going with vhyp for now just
because that's what VOF uses and I don't want to modify VOF to reuse it as
it is so I don't need to maintain a separate version and also get any
enhancements without further need to sync with spapr VOF.

I've found this document about possible hypercall interfaces on KVM (see
Hypercall ABIs at the end):


Having both ePAPR (1.) and PAPR (2.) hypercalls is a bit confusing. Does
vhyp correspond to 2. PAPR?


What's ePAPR then and how is it different from PAPR? I mean the acronym not the hypercall method, the latter is explained in that doc but what ePAPR stands for and why is that method called like that is not clear to me.

The ePAPR (1.) seems to be preferred by KVM and
MOL OSI supported for compatibility.

That document looks pretty out of date.  Most of it is only discussing
KVM PR, which is now barely maintained.  KVM HV only works with PAPR

The links says it's latest kernel docs, so maybe an update need to be sent to KVM?

So if we need something else instead of
2. PAPR hypercalls there seems to be two options: ePAPR and MOL OSI which
should work with KVM but then I'm not sure how to handle those on TCG.

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?

That's odd, cpu_stl() and memory_region_dispatch_write() should work
from board code (after the relevant memory regions are configured, of
course).  As an ISA serial port, it's probably accessed through IO
space, not memory space though, so you'd need &address_space_io.  And
if there is some bridge configuration then it's the bridge control
registers you need to look at not the serial registers - you'd have to
look at the bridge documentation for that.  Or, I guess the bridge
implementation in qemu, which you wrote part of.

I've found at last that stl_le_phys() works. There are so many of these that
I never know when to use which.

I think the address_space_rw calls in vof_client_call() in vof.c could also
use these for somewhat shorter code. I've ended up with
stl_le_phys(CPU(cpu)->as, addr, val) in my machine reset methodbut I don't
even need that now as it works without additional setup. Also VOF's memory
access is basically the same as the already existing rtas_st() and co. so
maybe that could be reused to make code smaller?

rtas_ld() and rtas_st() should only be used for reading/writing RTAS
parameters to and from memory.  Accessing IO shouldn't be done with

For IO you probably want the cpu_st*() variants in most cases, since
you're trying to emulate an IO access from the virtual cpu.

I think I've tried that but what worked to access mmio device registers are
stl_le_phys and similar that are wrappers around address_space_stl_*. But I
did not mean that for rtas_ld/_st but the part when vof accessing the
parameters passed by its hypercall which is memory access:


line 893, and vof_client_call before that is very similar to what h_rtas
does here:


and I also need to do the same for rtas in pegasos2 for which I'm just using
ldl_be_phys for now but I wonder if we really need 3 ways to do the same or
the rtas_ld/_st could be made more generic and reused here?

For your rtas implementation you could definitely re-use them.  For
the client call I'm a bit less confident, but if the in-guest-memory
structures are really the same, then it would make sense.

The memory structure seems very similar to me, the only difference is
calling the first field service in VOF instead of token in RTAS. Both are
just an array of big endian unit32_t with token, nargs, nret at the front
followed by args and rets. Since these rtas_ld/st are defined in spapr.h I
did not bother to split them off, so for pegasos2 rtas I'm just using the
ldl_be_* functions directly for which these are a shorthand for. If these
were split off for sharing between spapr rtas and VOF I may be able to reuse
them as well but it's not that important so just mentioned it as a possible
later clean up.

Ok, sounds reasonable to re-use them then, though maybe add an aliased
name for clarity ofci_{ld,st}(), maybe?  (for "Open Firmware Client

I'll wait for what Alexey decides to do in the next VOF patch version and if I can reuse that (I could if these were defined in vof.h). I don't want to come up with yet another abstraction to ldl_be_* which does not seem to make it more clear than using the actual functions for guest memory access which is what we're doing while getting the hypercall args so I think either using ldl_be_* directly or reusing already existing rfas_ls/_st would make sense but adding similar funcs with another name just makes it more confusing.


reply via email to

[Prev in Thread] Current Thread [Next in Thread]