> Thanks, I missed your original mail on this last year. I now opened an issue
> about this, with your suggested workaround:
> https://github.com/cloudius-systems/osv/issues/913
I saw, thanks.
> Given the vast number of compiler versions that people use, it's not
> surprising that once in a while we get a miscompiled
> and non-working OSv from one of them. Recently I worked around (see commit
> c38806f49cd2d0610227b62ee7170983684c6987)
> a bug which caused the round() function to hang in an infinite loop (which
> also caused the VM to hang while using all the CPU).
Understood - and I did see your round() fix and was hoping it fixed something
for me - but alas not.
> It would be nice if you could try whether other optimization options (e.g.,
> -O1) are enough to prevent this bug or -O0 is really
> needed. With some experimentation/printouts/debugging I guess you can also
> figure out what hangs, but I don't know how difficult
> it would be.
>
> In your original patch (see issue 913) you changed only one source file to be
> compiled without -O2, but in your latest patch, you
> changed more source files - was this necessary? In your original patch, you
> only changed compilation of lzloader.cc; This is a trivial
> source file, and I don't mind at all to permanently compile this specific
> file without optimization. However, the newer patch also changes the
> compilation of
> the uncompression implementation - this can have effect in boot performance,
> so if needed to change that too, we would need to
> measure its effect on boot speed.
It does indeed seem that the original patch is sufficient. It wasn’t at one
time, I’m sure - but for now I can use the single change for both debug and
release images.
$(call quiet, $(CXX) $(CXXFLAGS) -O0 -m32 -fno-instrument-functions -o $@
-c fastlz/lzloader.cc, CXX $<)
Its such a small file, but my assembler foo is very rusty. Attached are
objdumps of the two resulting compiles of lzloader.o, one with -O0 which
results in a working image, and one with -O1 with results in a hang as the VM
starts.
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build/release.x64/fastlz/lzloader.o: file format elf32-i386
Disassembly of section .text:
00000000 <memset>:
extern char _binary_loader_stripped_elf_lz_end;
extern char _binary_loader_stripped_elf_lz_size;
// std libraries used by fastlz.
extern "C" void *memset(void *s, int c, size_t n)
{
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 53 push %ebx
4: 83 ec 04 sub $0x4,%esp
7: e8 fc ff ff ff call 8 <memset+0x8>
c: 05 01 00 00 00 add $0x1,%eax
return __builtin_memset(s, c, n);
11: 83 ec 04 sub $0x4,%esp
14: ff 75 10 pushl 0x10(%ebp)
17: ff 75 0c pushl 0xc(%ebp)
1a: ff 75 08 pushl 0x8(%ebp)
1d: 89 c3 mov %eax,%ebx
1f: e8 fc ff ff ff call 20 <memset+0x20>
24: 83 c4 10 add $0x10,%esp
}
27: 8b 5d fc mov -0x4(%ebp),%ebx
2a: c9 leave
2b: c3 ret
0000002c <uncompress_loader>:
extern "C" void uncompress_loader()
{
2c: 55 push %ebp
2d: 89 e5 mov %esp,%ebp
2f: 53 push %ebx
30: 83 ec 04 sub $0x4,%esp
33: e8 fc ff ff ff call 34 <uncompress_loader+0x8>
38: 05 01 00 00 00 add $0x1,%eax
// pass a the last (maxout) parameter of fastlz_decompress. Let it
// uncompress as much as it has input. The Makefile already verifies
// that the uncompressed kernel doesn't overwrite this uncompression code.
// Sadly, "INT_MAX" is the largest number we can pass. If we ever need
// more than 2GB here, it won't work.
fastlz_decompress(&_binary_loader_stripped_elf_lz_start,
3d: 8b 90 00 00 00 00 mov 0x0(%eax),%edx
(size_t) &_binary_loader_stripped_elf_lz_size,
BUFFER_OUT, INT_MAX);
43: 68 ff ff ff 7f push $0x7fffffff
48: 68 00 00 20 00 push $0x200000
4d: 52 push %edx
4e: 8b 90 00 00 00 00 mov 0x0(%eax),%edx
54: 52 push %edx
55: 89 c3 mov %eax,%ebx
57: e8 fc ff ff ff call 58 <uncompress_loader+0x2c>
5c: 83 c4 10 add $0x10,%esp
}
5f: 90 nop
60: 8b 5d fc mov -0x4(%ebp),%ebx
63: c9 leave
64: c3 ret
Disassembly of section .text.__x86.get_pc_thunk.ax:
00000000 <__x86.get_pc_thunk.ax>:
{
0: 8b 04 24 mov (%esp),%eax
3: c3 ret
build/release.x64/fastlz/lzloader.o: file format elf32-i386
Disassembly of section .text:
00000000 <memset>:
extern char _binary_loader_stripped_elf_lz_end;
extern char _binary_loader_stripped_elf_lz_size;
// std libraries used by fastlz.
extern "C" void *memset(void *s, int c, size_t n)
{
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 53 push %ebx
4: 83 ec 08 sub $0x8,%esp
7: e8 fc ff ff ff call 8 <memset+0x8>
c: 81 c3 02 00 00 00 add $0x2,%ebx
return __builtin_memset(s, c, n);
12: ff 75 10 pushl 0x10(%ebp)
15: ff 75 0c pushl 0xc(%ebp)
18: ff 75 08 pushl 0x8(%ebp)
1b: e8 fc ff ff ff call 1c <memset+0x1c>
}
20: 8b 5d fc mov -0x4(%ebp),%ebx
23: c9 leave
24: c3 ret
00000025 <uncompress_loader>:
extern "C" void uncompress_loader()
{
25: 55 push %ebp
26: 89 e5 mov %esp,%ebp
28: 53 push %ebx
29: 83 ec 04 sub $0x4,%esp
2c: e8 fc ff ff ff call 2d <uncompress_loader+0x8>
31: 81 c3 02 00 00 00 add $0x2,%ebx
// that the uncompressed kernel doesn't overwrite this uncompression code.
// Sadly, "INT_MAX" is the largest number we can pass. If we ever need
// more than 2GB here, it won't work.
fastlz_decompress(&_binary_loader_stripped_elf_lz_start,
(size_t) &_binary_loader_stripped_elf_lz_size,
BUFFER_OUT, INT_MAX);
37: 68 ff ff ff 7f push $0x7fffffff
3c: 68 00 00 20 00 push $0x200000
41: ff b3 00 00 00 00 pushl 0x0(%ebx)
47: ff b3 00 00 00 00 pushl 0x0(%ebx)
4d: e8 fc ff ff ff call 4e <uncompress_loader+0x29>
}
52: 83 c4 10 add $0x10,%esp
55: 8b 5d fc mov -0x4(%ebp),%ebx
58: c9 leave
59: c3 ret
Disassembly of section .text.__x86.get_pc_thunk.bx:
00000000 <__x86.get_pc_thunk.bx>:
{
0: 8b 1c 24 mov (%esp),%ebx
3: c3 ret
Cheers,
Rick
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