Regression: Linux v5.15+ does not boot on Freescale P2020

[Christophe Leroy]

Le 26/07/2022 à 15:44, Segher Boessenkool a écrit :
> On Tue, Jul 26, 2022 at 11:02:59AM +0200, Arnd Bergmann wrote:
>> On Tue, Jul 26, 2022 at 10:34 AM Pali Rohár <pali at kernel.org> wrote:
>>> On Monday 25 July 2022 16:54:16 Segher Boessenkool wrote:
>>>> The EH field in larx insns is new since ISA 2.05, and some ISA 1.x cpu
>>>> implementations actually raise an illegal insn exception on EH=1.  It
>>>> appears P2020 is one of those.
>>>
>>> P2020 has e500 cores. e500 cores uses ISA 2.03. So this may be reason.
>>> But in official Freescale/NXP documentation for e500 is documented that
>>> lwarx supports also eh=1. Maybe it is not really supported.
>>> https://www.nxp.com/files-static/32bit/doc/ref_manual/EREF_RM.pdf (page 562)
> 
> (page 6-186)
> 
>>> At least there is NOTE:
>>> Some older processors may treat EH=1 as an illegal instruction.
> 
> And the architecture says
>    Programming Note
>    Warning: On some processors that comply with versions of the
>    architecture that precede Version 2.00, executing a Load And Reserve
>    instruction in which EH = 1 will cause the illegal instruction error
>    handler to be invoked.
> 
>> In commit d6ccb1f55ddf ("powerpc/85xx: Make sure lwarx hint isn't set on ppc32")
>> this was clarified to affect (all?) e500v1/v2,
> 
>    e500v1/v2 based chips will treat any reserved field being set in an
>    opcode as illegal.
> 
> while the architecture says
> 
>    Reserved fields in instructions are ignored by the processor.
> 
> Whoops :-)  We need fixes for processor implementation bugs all the
> time of course, but this is a massive *design* bug.  I'm surprised this
> CPU still works as well as it does!

"Programming Environments Manual for 32-Bit Implementations of the
PowerPC™ Architecture" §4.1.2.2.2 says: "Invalid forms result when a bit 
or operand is coded incorrectly, for example, or when a reserved bit 
(shown as ‘0’) is coded as ‘1’."

> 
> Even the venerable PEM (last updated in 1997) shows the EH field as
> reserved, always treated as 0.
> 
>> this one apparently
>> fixed it before,
>> but Christophe's commit effectively reverted that change.
>>
>> I think only the simple_spinlock.h file actually uses EH=1
> 
> That's right afaics.
> 
>> and this is not
>> included in non-SMP kernels, so presumably the only affected machines were
>> the rare dual-core e500v2 ones (p2020, MPC8572, bsc9132), which would
>> explain why nobody noticed for the past 9 months.
> 
> Also people using an SMP kernel on older cores should see the problem,
> no?  Or is that patched out?  Or does this use case never happen :-)

Maybe unlike e500, older cores ignore the EH bit and don't mind when 
it's set to 1 ?

Chritophe