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SubjectRe: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure
On Mon, Oct 16, 2017 at 2:18 PM, Boris Ostrovsky
<boris.ostrovsky@oracle.com> 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.
>
> Replacing the macro with
>
> #define PV_INDIRECT(addr) *addr // well, it's not so much
> indirect anymore
>
> makes things work. Or maybe it can be adjusted top be kept truly indirect.

That is still an indirect call, just using absolute addressing for the
pointer instead of RIP-relative. Alternatives has very limited
relocation capabilities. It will only handle a single call or jmp
replacement. Using absolute addressing is slightly less efficient
(takes one extra byte to encode, and needs a relocation for KASLR),
but it works just as well. You could also relocate the instruction
manually by adding the delta between the original and replacement code
to the displacement.

--
Brian Gerst

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