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SubjectRe: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure
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On 10/17/2017 01:24 AM, Josh Poimboeuf wrote:
> On Mon, Oct 16, 2017 at 02:18:48PM -0400, Boris Ostrovsky wrote:
>> On 10/12/2017 03:53 PM, Boris Ostrovsky wrote:
>>> On 10/12/2017 03:27 PM, Andrew Cooper wrote:
>>>> On 12/10/17 20:11, Boris Ostrovsky wrote:
>>>>> There is also another problem:
>>>>>
>>>>> [ 1.312425] general protection fault: 0000 [#1] SMP
>>>>> [ 1.312901] Modules linked in:
>>>>> [ 1.313389] CPU: 0 PID: 1 Comm: init Not tainted 4.14.0-rc4+ #6
>>>>> [ 1.313878] task: ffff88003e2c0000 task.stack: ffffc9000038c000
>>>>> [ 1.314360] RIP: 10000e030:entry_SYSCALL_64_fastpath+0x1/0xa5
>>>>> [ 1.314854] RSP: e02b:ffffc9000038ff50 EFLAGS: 00010046
>>>>> [ 1.315336] RAX: 000000000000000c RBX: 000055f550168040 RCX:
>>>>> 00007fcfc959f59a
>>>>> [ 1.315827] RDX: 0000000000000000 RSI: 0000000000000000 RDI:
>>>>> 0000000000000000
>>>>> [ 1.316315] RBP: 000000000000000a R08: 000000000000037f R09:
>>>>> 0000000000000064
>>>>> [ 1.316805] R10: 000000001f89cbf5 R11: ffff88003e2c0000 R12:
>>>>> 00007fcfc958ad60
>>>>> [ 1.317300] R13: 0000000000000000 R14: 000055f550185954 R15:
>>>>> 0000000000001000
>>>>> [ 1.317801] FS: 0000000000000000(0000) GS:ffff88003f800000(0000)
>>>>> knlGS:0000000000000000
>>>>> [ 1.318267] CS: e033 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>> [ 1.318750] CR2: 00007fcfc97ab218 CR3: 000000003c88e000 CR4:
>>>>> 0000000000042660
>>>>> [ 1.319235] Call Trace:
>>>>> [ 1.319700] Code: 51 50 57 56 52 51 6a da 41 50 41 51 41 52 41 53 48
>>>>> 83 ec 30 65 4c 8b 1c 25 c0 d2 00 00 41 f7 03 df 39 08 90 0f 85 a5 00 00
>>>>> 00 50 <ff> 15 9c 95 d0 ff 58 48 3d 4c 01 00 00 77 0f 4c 89 d1 ff 14 c5
>>>>> [ 1.321161] RIP: entry_SYSCALL_64_fastpath+0x1/0xa5 RSP: ffffc9000038ff50
>>>>> [ 1.344255] ---[ end trace d7cb8cd6cd7c294c ]---
>>>>> [ 1.345009] Kernel panic - not syncing: Attempted to kill init!
>>>>> exitcode=0x0000000b
>>>>>
>>>>>
>>>>> All code
>>>>> ========
>>>>> 0: 51 push %rcx
>>>>> 1: 50 push %rax
>>>>> 2: 57 push %rdi
>>>>> 3: 56 push %rsi
>>>>> 4: 52 push %rdx
>>>>> 5: 51 push %rcx
>>>>> 6: 6a da pushq $0xffffffffffffffda
>>>>> 8: 41 50 push %r8
>>>>> a: 41 51 push %r9
>>>>> c: 41 52 push %r10
>>>>> e: 41 53 push %r11
>>>>> 10: 48 83 ec 30 sub $0x30,%rsp
>>>>> 14: 65 4c 8b 1c 25 c0 d2 mov %gs:0xd2c0,%r11
>>>>> 1b: 00 00
>>>>> 1d: 41 f7 03 df 39 08 90 testl $0x900839df,(%r11)
>>>>> 24: 0f 85 a5 00 00 00 jne 0xcf
>>>>> 2a: 50 push %rax
>>>>> 2b:* ff 15 9c 95 d0 ff callq *-0x2f6a64(%rip) #
>>>>> 0xffffffffffd095cd <-- trapping instruction
>>>>> 31: 58 pop %rax
>>>>> 32: 48 3d 4c 01 00 00 cmp $0x14c,%rax
>>>>> 38: 77 0f ja 0x49
>>>>> 3a: 4c 89 d1 mov %r10,%rcx
>>>>> 3d: ff .byte 0xff
>>>>> 3e: 14 c5 adc $0xc5,%al
>>>>>
>>>>>
>>>>> so the original 'cli' was replaced with the pv call but to me the offset
>>>>> looks a bit off, no? Shouldn't it always be positive?
>>>> callq takes a 32bit signed displacement, so jumping back by up to 2G is
>>>> perfectly legitimate.
>>> Yes, but
>>>
>>> ostr@workbase> nm vmlinux | grep entry_SYSCALL_64_fastpath
>>> ffffffff817365dd t entry_SYSCALL_64_fastpath
>>> ostr@workbase> nm vmlinux | grep " pv_irq_ops"
>>> ffffffff81c2dbc0 D pv_irq_ops
>>> ostr@workbase>
>>>
>>> so pv_irq_ops.irq_disable is about 5MB ahead of where we are now. (I
>>> didn't mean that x86 instruction set doesn't allow negative
>>> displacement, I was trying to say that pv_irq_ops always live further down)
>> I believe the problem is this:
>>
>> #define PV_INDIRECT(addr) *addr(%rip)
>>
>> The displacement that the linker computes will be relative to the where
>> this instruction is placed at the time of linking, which is in
>> .pv_altinstructions (and not .text). So when we copy it into .text the
>> displacement becomes bogus.
> apply_alternatives() is supposed to adjust that displacement based on
> the new IP, though it could be messing that up somehow. (See patch
> 10/13.)
>

That patch doesn't take into account the fact that replacement
instructions may have to save/restore registers. So, for example,


- if (a->replacementlen && is_jmp(replacement[0]))
+ } else if (a->replacementlen == 6 && *insnbuf == 0xff &&
+ *(insnbuf+1) == 0x15) {
+ /* indirect call */
+ *(s32 *)(insnbuf + 2) += replacement - instr;
+ DPRINTK("Fix indirect CALL offset: 0x%x, CALL *0x%lx",
+ *(s32 *)(insnbuf + 2),
+ (unsigned long)instr + *(s32 *)(insnbuf + 2) + 6);
+

doesn't do the adjustment of

2a: 50 push %rax
2b:* ff 15 9c 95 d0 ff callq *-0x2f6a64(%rip)
31: 58 pop %rax

because instbuf points to 'push' and not to 'call'.

-boris

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