Hi Andreas,
On 02/02/16 04:39, Andreas Schwab wrote:
Greg Ungerer <gerg@xxxxxxxxxxx> writes:
The motivation for this change is a problem with non-MMU targets that
have broken signal return paths on newer versions of gcc. It appears as
though aliasing of the regs and switch stack pointers, caused by their
construction from pointers derived from the dummy long function parameter,
is resulting in the gcc optimizer removing what it thinks is useless
updates to the regs fields.
I wonder why gcc is doing that. The kernel is explicitly built without
strict aliasing, so this looks like something to investigate.
Agreed. My initial thoughts were this is a problem with gcc.
And I am not entirely sure it still isn't.
-asmlinkage int do_sigreturn(unsigned long __unused)
+asmlinkage int do_sigreturn(struct pt_regs *regs, struct switch_stack *sw)
Does it work to use this signature:
asmlinkage int do_sigreturn(struct switch_stack sw, struct pt_regs regs)
without changing the caller?
No, same problem. (I had to fix the use of regs inside the function,
but that was trivial). The generated code is essentially similar,
and large parts if it end up optimized away.
Original unmodified kernel code generates a do_sigreturn() as:
40021aaa <do_sigreturn>:
40021aaa: 4e56 ffd4 linkw %fp,#-44
40021aae: 2f0a movel %a2,%sp@-
40021ab0: 4e68 movel %usp,%a0
40021ab2: 2028 0018 movel %a0@(24),%d0
40021ab6: 2d40 ffd4 movel %d0,%fp@(-44)
40021aba: 2448 moveal %a0,%a2
40021abc: 2d68 0014 ffd8 movel %a0@(20),%fp@(-40)
40021ac2: 486e ffd4 pea %fp@(-44)
40021ac6: 4eb9 4002 f0b8 jsr 4002f0b8 <set_current_blocked>
40021acc: 200f movel %sp,%d0
40021ace: 0280 ffff e000 andil #-8192,%d0
40021ad4: 2240 moveal %d0,%a1
40021ad6: 203c 4002 f086 movel #1073934470,%d0
40021adc: 2051 moveal %a1@,%a0
40021ade: 2140 0154 movel %d0,%a0@(340)
40021ae2: 2d6a 0020 ffe4 movel %a2@(32),%fp@(-28)
40021ae8: 2d6a 001c ffe0 movel %a2@(28),%fp@(-32)
40021aee: 202e ffe4 movel %fp@(-28),%d0
40021af2: 206e ffe0 moveal %fp@(-32),%a0
40021af6: 4e60 movel %a0,%usp
40021af8: 246e ffd0 moveal %fp@(-48),%a2
40021afc: 588f addql #4,%sp
40021afe: 4e5e unlk %fp
40021b00: 4e75 rts
With your suggested change above it is generated as:
40021aaa <do_sigreturn>:
40021aaa: 4e56 ffd4 linkw %fp,#-44
40021aae: 2f0a movel %a2,%sp@-
40021ab0: 4e68 movel %usp,%a0
40021ab2: 2448 moveal %a0,%a2
40021ab4: 202a 0018 movel %a2@(24),%d0
40021ab8: 2d40 ffd4 movel %d0,%fp@(-44)
40021abc: 2d6a 0014 ffd8 movel %a2@(20),%fp@(-40)
40021ac2: 486e ffd4 pea %fp@(-44)
40021ac6: 4eb9 4002 f0b8 jsr 4002f0b8 <set_current_blocked>
40021acc: 200f movel %sp,%d0
40021ace: 0280 ffff e000 andil #-8192,%d0
40021ad4: 2240 moveal %d0,%a1
40021ad6: 203c 4002 f086 movel #1073934470,%d0
40021adc: 2051 moveal %a1@,%a0
40021ade: 2140 0154 movel %d0,%a0@(340)
40021ae2: 2d6a 001c ffe0 movel %a2@(28),%fp@(-32)
40021ae8: 2d6a 0020 ffe4 movel %a2@(32),%fp@(-28)
40021aee: 206e ffe0 moveal %fp@(-32),%a0
40021af2: 4e60 movel %a0,%usp
40021af4: 202e ffe4 movel %fp@(-28),%d0
40021af8: 588f addql #4,%sp
40021afa: 246e ffd0 moveal %fp@(-48),%a2
40021afe: 4e5e unlk %fp
40021b00: 4e75 rts
And finally with the patch I posted it generates:
40021aba <do_sigreturn>:
40021aba: 4e56 ffc0 linkw %fp,#-64
40021abe: 48d7 0c1c moveml %d2-%d4/%a2-%a3,%sp@
40021ac2: 246e 0008 moveal %fp@(8),%a2
40021ac6: 4e68 movel %usp,%a0
40021ac8: 2648 moveal %a0,%a3
40021aca: 202b 0018 movel %a3@(24),%d0
40021ace: 2d40 ffd4 movel %d0,%fp@(-44)
40021ad2: 2d6b 0014 ffd8 movel %a3@(20),%fp@(-40)
40021ad8: 486e ffd4 pea %fp@(-44)
40021adc: 4eb9 4002 f17c jsr 4002f17c <set_current_blocked>
40021ae2: 200f movel %sp,%d0
40021ae4: 0280 ffff e000 andil #-8192,%d0
40021aea: 2240 moveal %d0,%a1
40021aec: 203c 4002 f14a movel #1073934666,%d0
40021af2: 72ff moveq #-1,%d1
40021af4: 2051 moveal %a1@,%a0
40021af6: 2140 0154 movel %d0,%a0@(340)
40021afa: 2d6b 0020 ffe4 movel %a3@(32),%fp@(-28)
40021b00: 2d6b 001c ffe0 movel %a3@(28),%fp@(-32)
40021b06: 2d6b 0024 ffe8 movel %a3@(36),%fp@(-24)
40021b0c: 2d6b 0028 ffec movel %a3@(40),%fp@(-20)
40021b12: 202e ffe4 movel %fp@(-28),%d0
40021b16: 206e ffe0 moveal %fp@(-32),%a0
40021b1a: 2d6b 002c fff0 movel %a3@(44),%fp@(-16)
40021b20: 2d6b 0034 fff8 movel %a3@(52),%fp@(-8)
40021b26: 2d6b 0038 fffc movel %a3@(56),%fp@(-4)
40021b2c: 24ae ffe8 movel %fp@(-24),%a2@
40021b30: 256e ffec 0014 movel %fp@(-20),%a2@(20)
40021b36: 256e fff0 0018 movel %fp@(-16),%a2@(24)
40021b3c: 256e fffa 0030 movel %fp@(-6),%a2@(48)
40021b42: 156e fff9 002f moveb %fp@(-7),%a2@(47)
40021b48: 2540 0020 movel %d0,%a2@(32)
40021b4c: 2541 0024 movel %d1,%a2@(36)
40021b50: 4e60 movel %a0,%usp
40021b52: 780f moveq #15,%d4
40021b54: 588f addql #4,%sp
40021b56: 322e fffe movew %fp@(-2),%d1
40021b5a: 162a 002c moveb %a2@(44),%d3
40021b5e: 2401 movel %d1,%d2
40021b60: e08a lsrl #8,%d2
40021b62: c684 andl %d4,%d3
40021b64: 0282 0000 00f0 andil #240,%d2
40021b6a: 0281 0000 0fff andil #4095,%d1
40021b70: 8483 orl %d3,%d2
40021b72: 1542 002c moveb %d2,%a2@(44)
40021b76: 342a 002c movew %a2@(44),%d2
40021b7a: 0282 ffff f000 andil #-4096,%d2
40021b80: 8282 orl %d2,%d1
40021b82: 3541 002c movew %d1,%a2@(44)
40021b86: 4cee 0c1c ffc0 moveml %fp@(-64),%d2-%d4/%a2-%a3
40021b8c: 4e5e unlk %fp
40021b8e: 4e75 rts
The only generated code that works of these is last one -
with my patch applied.
Regards
Greg
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