Hi Borislav, thank you for testing and finding it. On Fri, Jan 9, 2015 at 1:19 PM, Borislav Petkov <[email protected]> wrote: > Hmm, this patch breaks booting my kvm guest: it stops booting at some > point and restarts itself after a couple of seconds. > > The monitor says rIP points to ffffffff8167ae30 which is this: > > ffffffff8167ae30 <async_page_fault>: > ffffffff8167ae30: ff 15 fa 62 31 00 callq *0x3162fa(%rip) > # ffffffff81991130 <pv_irq_ops+0x30> > ffffffff8167ae36: 48 83 ec 78 sub $0x78,%rsp > ffffffff8167ae3a: e8 d1 01 00 00 callq ffffffff8167b010 > <error_entry> > ffffffff8167ae3f: 48 89 e7 mov %rsp,%rdi > ffffffff8167ae42: 48 8b 74 24 78 mov 0x78(%rsp),%rsi > ffffffff8167ae47: 48 c7 44 24 78 ff ff movq > $0xffffffffffffffff,0x78(%rsp) > ffffffff8167ae4e: ff ff > ffffffff8167ae50: e8 9b 9e 9c ff callq ffffffff81044cf0 > <do_async_page_fault> > ffffffff8167ae55: e9 76 02 00 00 jmpq ffffffff8167b0d0 > <error_exit> > ffffffff8167ae5a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
I just looked at disassembly of thunk_64.o before and after the patch. Here's what I see: Before: Disassembly of section .text: 0000000000000000 <restore-0x30>: 0: 48 83 ec 48 sub $0x48,%rsp 4: 48 89 7c 24 40 mov %rdi,0x40(%rsp) 9: 48 89 74 24 38 mov %rsi,0x38(%rsp) e: 48 89 54 24 30 mov %rdx,0x30(%rsp) 13: 48 89 4c 24 28 mov %rcx,0x28(%rsp) 18: 48 89 44 24 20 mov %rax,0x20(%rsp) 1d: 4c 89 44 24 18 mov %r8,0x18(%rsp) 22: 4c 89 4c 24 10 mov %r9,0x10(%rsp) 27: 4c 89 54 24 08 mov %r10,0x8(%rsp) 2c: 4c 89 1c 24 mov %r11,(%rsp) 0000000000000030 <restore>: 30: 4c 8b 1c 24 mov (%rsp),%r11 34: 4c 8b 54 24 08 mov 0x8(%rsp),%r10 39: 4c 8b 4c 24 10 mov 0x10(%rsp),%r9 3e: 4c 8b 44 24 18 mov 0x18(%rsp),%r8 43: 48 8b 44 24 20 mov 0x20(%rsp),%rax 48: 48 8b 4c 24 28 mov 0x28(%rsp),%rcx 4d: 48 8b 54 24 30 mov 0x30(%rsp),%rdx 52: 48 8b 74 24 38 mov 0x38(%rsp),%rsi 57: 48 8b 7c 24 40 mov 0x40(%rsp),%rdi 5c: 48 83 c4 48 add $0x48,%rsp 60: c3 retq After: Disassembly of section .text: 0000000000000000 <restore>: 0: 4c 8b 1c 24 mov (%rsp),%r11 4: 4c 8b 54 24 08 mov 0x8(%rsp),%r10 9: 4c 8b 4c 24 10 mov 0x10(%rsp),%r9 e: 4c 8b 44 24 18 mov 0x18(%rsp),%r8 13: 48 8b 44 24 20 mov 0x20(%rsp),%rax 18: 48 8b 4c 24 28 mov 0x28(%rsp),%rcx 1d: 48 8b 54 24 30 mov 0x30(%rsp),%rdx 22: 48 8b 74 24 38 mov 0x38(%rsp),%rsi 27: 48 8b 7c 24 40 mov 0x40(%rsp),%rdi 2c: 48 03 24 25 48 00 00 add 0x48,%rsp 33: 00 34: c3 retq IOW, my patch, on the level of generated assembly, results only in removal of unreachable "SAVE_ARGS" thing. I looked into git history all the way back to 2005. The part + /* SAVE_ARGS below is used only for the .cfi directives it contains. */ + CFI_STARTPROC + SAVE_ARGS +restore: was there in the very first git commit. I don't see how this SAVE_ARGS can affect anything. It *is* unreachable, right? Does kvm guest code really parse and use CFI data in its operation? That's the only way the breakage can be explained. In order to narrow it down, can you, instead of my patch, try just deleting this one line, and see whether breakage appears? Thanks! -- vda -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
