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    • Fangrui Song's avatar
      powerpc: Add "-z notext" flag to disable diagnostic · 03557853
      Fangrui Song authored 
      Object files used to link .tmp_vmlinux.kallsyms1 have many
R_PPC64_ADDR64 relocations in non-SHF_WRITE sections. There are many
text relocations (e.g. in .rela___ksymtab_gpl+* and .rela__mcount_loc
sections) in a -pie link and are disallowed by LLD:

  ld.lld: error: can't create dynamic relocation R_PPC64_ADDR64 against local symbol in readonly segment; recompile object files with -fPIC or pass '-Wl,-z,notext' to allow text relocations in the output
  >>> defined in arch/powerpc/kernel/head_64.o
  >>> referenced by arch/powerpc/kernel/head_64.o:(__restart_table+0x10)

Newer GNU ld configured with "--enable-textrel-check=error" will report
an error as well:

  $ ld-new -EL -m elf64lppc -pie ... -o .tmp_vmlinux.kallsyms1 ...
  ld-new: read-only segment has dynamic relocations

Add "-z notext" to suppress the errors. Non-CONFIG_RELOCATABLE builds
use the default -no-pie mode and thus R_PPC64_ADDR64 relocations can be
resolved at link-time.

Reported-by: It...
      03557853
    • Christophe Leroy's avatar
      powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with asm goto · 1e688dd2
      Christophe Leroy authored 
      

Using asm goto in __WARN_FLAGS() and WARN_ON() allows more
flexibility to GCC.

For that add an entry to the exception table so that
program_check_exception() knowns where to resume execution
after a WARNING.

Here are two exemples. The first one is done on PPC32 (which
benefits from the previous patch), the second is on PPC64.

	unsigned long test(struct pt_regs *regs)
	{
		int ret;

		WARN_ON(regs->msr & MSR_PR);

		return regs->gpr[3];
	}

	unsigned long test9w(unsigned long a, unsigned long b)
	{
		if (WARN_ON(!b))
			return 0;
		return a / b;
	}

Before the patch:

	000003a8 <test>:
	 3a8:	81 23 00 84 	lwz     r9,132(r3)
	 3ac:	71 29 40 00 	andi.   r9,r9,16384
	 3b0:	40 82 00 0c 	bne     3bc <test+0x14>
	 3b4:	80 63 00 0c 	lwz     r3,12(r3)
	 3b8:	4e 80 00 20 	blr

	 3bc:	0f e0 00 00 	twui    r0,0
	 3c0:	80 63 00 0c 	lwz     r3,12(r3)
	 3c4:	4e 80 00 20 	blr

	0000000000000bf0 <.test9w>:
	 bf0:	7c 89 00 74 	cntlzd  r9,r4
	 bf4:	79 29 d1 82 	rldicl  r9,r9,58,6
	 bf8:	0b 09 00 00 	tdnei   r9,0
	 bfc:	2c 24 00 00 	cmpdi   r4,0
	 c00:	41 82 00 0c 	beq     c0c <.test9w+0x1c>
	 c04:	7c 63 23 92 	divdu   r3,r3,r4
	 c08:	4e 80 00 20 	blr

	 c0c:	38 60 00 00 	li      r3,0
	 c10:	4e 80 00 20 	blr

After the patch:

	000003a8 <test>:
	 3a8:	81 23 00 84 	lwz     r9,132(r3)
	 3ac:	71 29 40 00 	andi.   r9,r9,16384
	 3b0:	40 82 00 0c 	bne     3bc <test+0x14>
	 3b4:	80 63 00 0c 	lwz     r3,12(r3)
	 3b8:	4e 80 00 20 	blr

	 3bc:	0f e0 00 00 	twui    r0,0

	0000000000000c50 <.test9w>:
	 c50:	7c 89 00 74 	cntlzd  r9,r4
	 c54:	79 29 d1 82 	rldicl  r9,r9,58,6
	 c58:	0b 09 00 00 	tdnei   r9,0
	 c5c:	7c 63 23 92 	divdu   r3,r3,r4
	 c60:	4e 80 00 20 	blr

	 c70:	38 60 00 00 	li      r3,0
	 c74:	4e 80 00 20 	blr

In the first exemple, we see GCC doesn't need to duplicate what
happens after the trap.

In the second exemple, we see that GCC doesn't need to emit a test
and a branch in the likely path in addition to the trap.

We've got some WARN_ON() in .softirqentry.text section so it needs
to be added in the OTHER_TEXT_SECTIONS in modpost.c

Signed-off-by: default avatarChristophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/389962b1b702e3c78d169e59bcfac56282889173.1618331882.git.christophe.leroy@csgroup.eu
      1e688dd2
  15. Aug 14, 2021
    • Christophe Leroy's avatar
      powerpc/bug: Remove specific powerpc BUG_ON() and WARN_ON() on PPC32 · db87a719
      Christophe Leroy authored 
      powerpc BUG_ON() and WARN_ON() are based on using twnei instruction.

For catching simple conditions like a variable having value 0, this
is efficient because it does the test and the trap at the same time.
But most conditions used with BUG_ON or WARN_ON are more complex and
forces GCC to format the condition into a 0 or 1 value in a register.
This will usually require 2 to 3 instructions.

The most efficient solution would be to use __builtin_trap() because
GCC is able to optimise the use of the different trap instructions
based on the requested condition, but this is complex if not
impossible for the following reasons:
- __builtin_trap() is a non-recoverable instruction, so it can't be
used for WARN_ON
- Knowing which line of code generated the trap would require the
analysis of DWARF information. This is not a feature we have today.

As mentioned in commit 8d4fbcfb

 ("Fix WARN_ON() on bitfield ops")
the way WARN_ON() is implemented is suboptimal. That commit also
mentions an issue with 'long long' condition. It fixed it for
WARN_ON() but the same problem still exists today with BUG_ON() on
PPC32. It will be fixed by using the generic implementation.

By using the generic implementation, gcc will naturally generate a
branch to the unconditional trap generated by BUG().

As modern powerpc implement zero-cycle branch,
that's even more efficient.

And for the functions using WARN_ON() and its return, the test
on return from WARN_ON() is now also used for the WARN_ON() itself.

On PPC64 we don't want it because we want to be able to use CFAR
register to track how we entered the code that trapped. The CFAR
register would be clobbered by the branch.

A simple test function:

	unsigned long test9w(unsigned long a, unsigned long b)
	{
		if (WARN_ON(!b))
			return 0;
		return a / b;
	}

Before the patch:

	0000046c <test9w>:
	 46c:	7c 89 00 34 	cntlzw  r9,r4
	 470:	55 29 d9 7e 	rlwinm  r9,r9,27,5,31
	 474:	0f 09 00 00 	twnei   r9,0
	 478:	2c 04 00 00 	cmpwi   r4,0
	 47c:	41 82 00 0c 	beq     488 <test9w+0x1c>
	 480:	7c 63 23 96 	divwu   r3,r3,r4
	 484:	4e 80 00 20 	blr

	 488:	38 60 00 00 	li      r3,0
	 48c:	4e 80 00 20 	blr

After the patch:

	00000468 <test9w>:
	 468:	2c 04 00 00 	cmpwi   r4,0
	 46c:	41 82 00 0c 	beq     478 <test9w+0x10>
	 470:	7c 63 23 96 	divwu   r3,r3,r4
	 474:	4e 80 00 20 	blr

	 478:	0f e0 00 00 	twui    r0,0
	 47c:	38 60 00 00 	li      r3,0
	 480:	4e 80 00 20 	blr

So we see before the patch we need 3 instructions on the likely path
to handle the WARN_ON(). With the patch the trap goes on the unlikely
path.

See below the difference at the entry of system_call_exception where
we have several BUG_ON(), allthough less impressing.

With the patch:

	00000000 <system_call_exception>:
	   0:	81 6a 00 84 	lwz     r11,132(r10)
	   4:	90 6a 00 88 	stw     r3,136(r10)
	   8:	71 60 00 02 	andi.   r0,r11,2
	   c:	41 82 00 70 	beq     7c <system_call_exception+0x7c>
	  10:	71 60 40 00 	andi.   r0,r11,16384
	  14:	41 82 00 6c 	beq     80 <system_call_exception+0x80>
	  18:	71 6b 80 00 	andi.   r11,r11,32768
	  1c:	41 82 00 68 	beq     84 <system_call_exception+0x84>
	  20:	94 21 ff e0 	stwu    r1,-32(r1)
	  24:	93 e1 00 1c 	stw     r31,28(r1)
	  28:	7d 8c 42 e6 	mftb    r12
	...
	  7c:	0f e0 00 00 	twui    r0,0
	  80:	0f e0 00 00 	twui    r0,0
	  84:	0f e0 00 00 	twui    r0,0

Without the patch:

	00000000 <system_call_exception>:
	   0:	94 21 ff e0 	stwu    r1,-32(r1)
	   4:	93 e1 00 1c 	stw     r31,28(r1)
	   8:	90 6a 00 88 	stw     r3,136(r10)
	   c:	81 6a 00 84 	lwz     r11,132(r10)
	  10:	69 60 00 02 	xori    r0,r11,2
	  14:	54 00 ff fe 	rlwinm  r0,r0,31,31,31
	  18:	0f 00 00 00 	twnei   r0,0
	  1c:	69 60 40 00 	xori    r0,r11,16384
	  20:	54 00 97 fe 	rlwinm  r0,r0,18,31,31
	  24:	0f 00 00 00 	twnei   r0,0
	  28:	69 6b 80 00 	xori    r11,r11,32768
	  2c:	55 6b 8f fe 	rlwinm  r11,r11,17,31,31
	  30:	0f 0b 00 00 	twnei   r11,0
	  34:	7d 8c 42 e6 	mftb    r12

Signed-off-by: default avatarChristophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b286e07fb771a664b631cd07a40b09c06f26e64b.1618331881.git.christophe.leroy@csgroup.eu
      db87a719
  17. Aug 13, 2021