The "bl __restartkernel" patch: I've no clue why I've not seen the problem

> The branch main has been updated by bdragon:
> 
> URL: 
> https://cgit.FreeBSD.org/src/commit/?id=d26f2a50ff48dacd38ba358d658882d51f7bdbc4
> 
> 
> commit d26f2a50ff48dacd38ba358d658882d51f7bdbc4
> Author:     Brandon Bergren <
> bdragon at FreeBSD.org
> >
> AuthorDate: 2021-02-07 22:05:41 +0000
> Commit:     Brandon Bergren <
> bdragon at FreeBSD.org
> >
> CommitDate: 2021-02-07 22:13:55 +0000
> 
>     powerpc64: Fix boot on virtual-mode OF (PowerMac G5)
>     
>     In 78599c32efed3247d165302a1fbe8d9203e38974, CFI endproc decoration was
>     added to locore64.S. However, it missed the subtle detail that
>     __restartkernel_virtual() falls through to __restartkernel(). This was
>     causing boot failure on PowerMac G5, as it tried to execute the
>     epilogue as code.
>     
>     Fix this by branching to __restartkernel() instead of intentionally
>     running off the end of the function.
>     
>     While here, add some additional notes on how the virtual mode restart
>     works.
>     
>     MFC after:      3 days
> ---
>  sys/powerpc/aim/locore64.S | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/sys/powerpc/aim/locore64.S b/sys/powerpc/aim/locore64.S
> index 0bc0619620d9..f0a183c4b331 100644
> --- a/sys/powerpc/aim/locore64.S
> +++ b/sys/powerpc/aim/locore64.S
> @@ -255,6 +255,16 @@ ASENTRY_NOPROF(__restartkernel_virtual)
>  	addi	%r14, %r14, 1
>  	cmpdi	%r14, 16
>  	blt	1b
> +
> +	/*
> +	 * Now that we are set up with a temporary direct map, we can
> +	 * continue with __restartkernel. Translation will be switched
> +	 * back on at the rfid, at which point we will be executing from
> +	 * the temporary direct map we just installed, until the kernel
> +	 * takes over responsibility for the MMU.
> +	 */
> +	bl	__restartkernel
> +	nop
>  ASEND(__restartkernel_virtual)
>  
>  ASENTRY_NOPROF(__restartkernel)

Wow. Somehow I've not had boot powerpc64 FreeBSD boot
problems since the ASEND(__restartkernel_virtual)
addition. For example, the current boot code has
( from objdump output of /boot/kernel/kernel ):

0000000000100710 <__restartkernel_virtual> addis   r2,r12,302
0000000000100714 <__restartkernel_virtual+0x4> addi    r2,r2,10480
0000000000100718 <__restartkernel_virtual+0x8> mfmsr   r14
000000000010071c <__restartkernel_virtual+0xc> andi.   r14,r14,61389
0000000000100720 <__restartkernel_virtual+0x10> mtmsr   r14
0000000000100724 <__restartkernel_virtual+0x14> isync
0000000000100728 <__restartkernel_virtual+0x18> li      r14,0
000000000010072c <__restartkernel_virtual+0x1c> li      r18,0
0000000000100730 <__restartkernel_virtual+0x20> oris    r18,r18,49152
0000000000100734 <__restartkernel_virtual+0x24> rldicr  r18,r18,32,31
0000000000100738 <__restartkernel_virtual+0x28> slbmfev r15,r14
000000000010073c <__restartkernel_virtual+0x2c> slbmfee r16,r14
0000000000100740 <__restartkernel_virtual+0x30> or      r16,r16,r14
0000000000100744 <__restartkernel_virtual+0x34> ori     r16,r16,16
0000000000100748 <__restartkernel_virtual+0x38> or      r16,r16,r18
000000000010074c <__restartkernel_virtual+0x3c> rldicr  r17,r16,0,37
0000000000100750 <__restartkernel_virtual+0x40> isync
0000000000100754 <__restartkernel_virtual+0x44> slbie   r17
0000000000100758 <__restartkernel_virtual+0x48> slbmte  r15,r16
000000000010075c <__restartkernel_virtual+0x4c> isync
0000000000100760 <__restartkernel_virtual+0x50> addi    r14,r14,1
0000000000100764 <__restartkernel_virtual+0x54> cmpdi   r14,16
0000000000100768 <__restartkernel_virtual+0x58> blt     0000000000100738 <__restartkernel_virtual+0x28>
        ...
0000000000100778 <__restartkernel_virtual+0x68> nop
000000000010077c <__restartkernel_virtual+0x6c> nop
0000000000100780 <__restartkernel> addis   r2,r12,302
0000000000100784 <__restartkernel+0x4> addi    r2,r2,10368
0000000000100788 <__restartkernel+0x8> mtsrr1  r9
000000000010078c <__restartkernel+0xc> bl      0000000000100790 <__restartkernel+0x10>
0000000000100790 <__restartkernel+0x10> mflr    r25
0000000000100794 <__restartkernel+0x14> add     r25,r8,r25
0000000000100798 <__restartkernel+0x18> addi    r25,r25,20
000000000010079c <__restartkernel+0x1c> mtsrr0  r25
00000000001007a0 <__restartkernel+0x20> rfid
00000000001007a4 <__restartkernel+0x24> bl      0000000000100630 <__start>
00000000001007a8 <__restartkernel+0x28> nop

I wonder what implicit oddities have been happening
because of it, without obvious boot problems. Or is
the above only involved for some PowerMac G5s?

Cool find.

I've a ports rebuild in progress that may well not
finish for another 6+ hours. I'll not be updating
for this until sometime after the port build
finishes.

===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)

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