[SLOF] [PATCH] OF: Add a separate direct kernel loading word

[Alexey Kardashevskiy]

On 7/12/22 23:48, Segher Boessenkool wrote:
> Hi!
> 
> On Tue, Jul 12, 2022 at 10:46:24AM +1000, Jordan Niethe wrote:
>> Currently, go-64 is used for booting a kernel from qemu (i.e. -kernel).
>> However, there is an expectation from users that this should be able to
>> boot not just vmlinux kernels but things like Zimages too.
>>
>> The bootwrapper of a BE zImage is a 32-bit ELF. Attempting to load that
>> with go-64 means that it will be ran with MSR_SF set (64-bit mode). This
>> crashes early in boot (usually due to what should be 32-bit operations
>> being done with 64-bit registers eventually leading to an incorrect
>> address being generated and branched to).
> 
> In PowerPC, all operations are done the same in SF=0 and SF=1 modes,
> except:
>    - For addressing storage, the high 32 bits are ignored if SF=0;
>    - For bdz and bdnz (bc with BO[2]=1) the high 32 bits are ignored;
>    - For integer record form instructions ("dot instructions"), the high
>      32 bits are ignored.
> Everything else is done exactly the same with SF=0 -- the high 32 bits
> of everything are set in exactly the same way, for example.
> 
> In practice, what bites you is the first item, when doing table jumps:
> it ends up jumping to 0xffffffff12345678 instead of 0x0000000012345678
> when running 32-bit code with SF=1.  You get about two million
> instructions into yaboot before it blows up, for example :-)

It crashes lot sooner with a BE zImage :)

>  Dot insns
> are very common, but you do end up with the result properly (sign-)
> extended most of the time :-)
> 
>> Note that our 64-bit payloads are prepared to enter with MSR_SF cleared
>> and set it themselves very early.
>>
>> Add a new word named go-direct that will execute any simple payload
>> in-place and will enter with MSR_SF cleared. This allows booting a BE
>> zImage from qemu with -machine kernel-addr=0.
> 
> Ouch.  So you run 64-bit programs in 32-bit mode as well, just hoping
> they will deal with it?  Not a good idea :-(  Current Linux is fine with
> it, but are other payloads, including future Linux?

https://www.devicetree.org/open-firmware/bindings/ppc/release/ppc-2_1.html 
says "Upon entry to the client program, the following registers shall 
contain the following values: ... msr ... SF=0, 32-bit mode". I'd think 
that it is up to that client program to adjust MSR_SF, not the firmware.

> "init-program" is supposed to set the MSR state correctly (in ciregs
>> srr1), based on the ELF headers (and btw the same is true for the LE
> flag etc).  A little ELF parsing is needed.


This is the case when QEMU runs with "-kernel zImage" - QEMU loads the 
ELF and SLOF has no idea what was the binary. QEMU does store a 
"/chosen/qemu,boot-kernel-le" property but there is nothing for 32bit.

> 
> Hope this helps,

Always does! :)

> 
> 
> Segher
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-- 
Alexey