Re: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure

[Brian Gerst]

On Mon, Oct 16, 2017 at 2:18 PM, Boris Ostrovsky
<boris.ostrovsky@xxxxxxxxxx> 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