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Re: [RFC PATCH 0/2] QEMU/openbios: PPC Software TLB support in the G4 fa

From: Fabiano Rosas
Subject: Re: [RFC PATCH 0/2] QEMU/openbios: PPC Software TLB support in the G4 family
Date: Fri, 26 Nov 2021 09:31:11 -0300

Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> writes:

> On 26/11/2021 08:40, Cédric Le Goater wrote:
>> On 11/26/21 09:01, Mark Cave-Ayland wrote:
>>> On 24/11/2021 22:00, Fabiano Rosas wrote:
>>>> Fabiano Rosas <farosas@linux.ibm.com> writes:
>>>>> Hi all,
>>>>> We have this bug in QEMU which indicates that we haven't been able to
>>>>> run openbios on a 7450 cpu for quite a long time:
>>>>> https://gitlab.com/qemu-project/qemu/-/issues/86
>>>>> OK:
>>>>>    $ ./qemu-system-ppc -serial mon:stdio -nographic -cpu 7410
>>>>>    >> =============================================================
>>>>>    >> OpenBIOS 1.1 [Nov 1 2021 20:36]
>>>>>    ...
>>>>> NOK:
>>>>>    $ ./qemu-system-ppc -serial mon:stdio -nographic -cpu 7450 -d int
>>>>>    Raise exception at fff08cc4 => 0000004e (00)
>>>>>    QEMU: Terminated
>>>>> The actual issue is straightforward. There is a non-architected
>>>>> feature that QEMU has enabled by default that openbios doesn't know
>>>>> about. From the user manual:
>>>>> "The MPC7540 has a set of implementation-specific registers,
>>>>> exceptions, and instructions that facilitate very efficient software
>>>>> searching of the page tables in memory for when software table
>>>>> searching is enabled (HID0[STEN] = 1). This section describes those
>>>>> resources and provides three example code sequences that can be used
>>>>> in a MPC7540 system for an efficient search of the translation tables
>>>>> in software. These three code sequences can be used as handlers for
>>>>> the three exceptions requiring access to the PTEs in the page tables
>>>>> in memory in this case-instruction TLB miss, data TLB miss on load,
>>>>> and data TLB miss on store exceptions."
>>>>> The current state:
>>>>> 1) QEMU does not check HID0[STEN] and makes the feature always enabled
>>>>> by setting these cpus with the POWERPC_MMU_SOFT_74xx MMU model,
>>>>> instead of the generic POWERPC_MMU_32B.
>>>>> 2) openbios does not recognize the PVRs for those cpus and also does
>>>>> not have any handlers for the software TLB exceptions (vectors 0x1000,
>>>>> 0x1100, 0x1200).
>>>>> Some assumptions (correct me if I'm wrong please):
>>>>> - openbios is the only firmware we use for the following cpus: 7441,
>>>>> 7445, 7450, 7451, 7455, 7457, 7447, 7447a, 7448.
>>>>> - without openbios, we cannot have a guest running on these cpus.
>>>>> So to bring 7450 back to life we would need to either:
>>>>> a) find another firmware/guest OS code that supports the feature;
>>>>> b) implement the switching of the feature in QEMU and have the guest
>>>>> code enable it only when supported. That would take some fiddling with
>>>>> the MMU code to: merge POWERPC_MMU_SOFT_74xx into POWERPC_MMU_32B,
>>>>> check the HID0[STEN] bit, figure out how to switch from HW TLB miss to
>>>>> SW TLB miss on demand, block access to the TLBMISS register (and
>>>>> others) when the feature is off, and so on;
>>>>> c) leave the feature enabled in QEMU and implement the software TLB
>>>>> miss handlers in openbios. The UM provides sample code, so this is
>>>>> easy;
>>>>> d) remove support for software TLB search for the 7450 family and
>>>>> switch the cpus to the POWERPC_MMU_32B model. This is by far the
>>>>> easiest solution, but could cause problems for any (which?) guest OS
>>>>> code that actually uses the feature. All of the existing code for the
>>>>> POWERPC_MMU_SOFT_74xx MMU model would probably be removed since it
>>>>> would be dead code then;
>>>>> Option (c) seemed to me like a good compromise so this is a patch
>>>>> series for openbios doing that and also adding the necessary PVRs so
>>>>> we can get a working guest with these cpus without too much effort.
>>>>> I have also a patch for QEMU adding basic sanity check tests for the
>>>>> 7400 and 7450 families. I'll send that separately to the QEMU ml.
>>>>> Fabiano Rosas (2):
>>>>>    ppc: Add support for MPC7450 software TLB miss interrupts
>>>>>    ppc: Add PVRs for the MPC7450 family
>>>>>   arch/ppc/qemu/init.c  |  52 ++++++++++
>>>>>   arch/ppc/qemu/start.S | 236 +++++++++++++++++++++++++++++++++++++++++-
>>>>>   2 files changed, 285 insertions(+), 3 deletions(-)
>>>> (Adding Mark because his email got somehow dropped from the original
>>>> message)
>>>> So with these patches in OpenBIOS we could get a bit further and call
>>>> into the Linux kernel using the same image as the one used for the
>>>> 7400. However there seems to be no support for the 7450 software TLB in
>>>> the kernel. There are only handlers for the 4xx, 8xx and 603 which are
>>>> different code altogether. There's no mention of the TLBMISS and
>>>> PTEHI/LO registers in the code as well.
>>>> Do we know of any guest OS that implements the 7450 software TLB at
>>>> vectors 0x1000, 0x1100 and 0x1200? Otherwise replacing the
>>>> POWERPC_MMU_SOFT_74xx model with POWERPC_MMU_32B might be the only way
>>>> of getting an OS to run in the 7450 family.
>>> My experience of anything other than the default CPUs used on the PPC Mac 
>>> machines 
>>> is basically zero, so you're certainly in new territory :)
>>> I could live with your proposed solution c) although it would be nice to 
>>> guard the 
>>> extra vectors so that they remain uninitialised for the non-7450 CPUs. My 
>>> main 
>>> question is if the kernel itself doesn't support software TLBs then does 
>>> adding the 
>>> new code help at all? 

The point of c) is to allow _something_ to run on the CPU with software
TLB enabled. I think that would be enough to make sure the code doesn't
rot too much. So more of a maintenance concern.

About improving the OpenBIOS patch, no worries, I can certainly do that
once we define a course of action.

>> yes, it helps to boot Linux and MacOS (9 and 10) on those CPUs but you still
>> need to replace the mmu model to POWERPC_MMU_32B in QEMU.
>>> Or are you eventually planning for solution b) to improve QEMU's 7450 CPU 
>>> emulation 
>>> for developers without real hardware?
>> b) would be nice to have but since we don't have any images using it, may
>> be it's time to drop support from QEMU.
> Right. If we're doing this to say "I can boot a kernel with a 7450 cpu in 
> QEMU" but 
> the implementation is different from real hardware, then I'm not sure what 
> the real 
> value is. That effectively leaves option b) if someone is willing to do the 
> work, or 
> as you say to simply remove the code from QEMU.

Yeah, that is a good point. Although the software TLB is well contained,
so we could certainly document that our 7450s don't have that feature
and call it a day. Does QEMU have any policy on how much of a machine is
required to be implemented?

I am more inclined to apply c) for now as I said, just to have some code
running on the CPU and maybe document in a gitlab issue that we're
lacking the runtime switch and eventually implement that. It's not like
this is high traffic code anyway. It has been broken for 10+ years.

That said, if Cédric and Daniel see more value in moving the 7450s to
the POWERPC_MMU_32B I won't oppose.

> ATB,
> Mark.

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