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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 10/06/2017 10:32 AM, Josh Poimboeuf wrote:
> On Thu, Oct 05, 2017 at 04:35:03PM -0400, Boris Ostrovsky wrote:
>>>  #ifdef CONFIG_PARAVIRT
>>> +/*
>>> + * Paravirt alternatives are applied much earlier than normal alternatives.
>>> + * They are only applied when running on a hypervisor.  They replace some
>>> + * native instructions with calls to pv ops.
>>> + */
>>> +void __init apply_pv_alternatives(void)
>>> +{
>>> +   setup_force_cpu_cap(X86_FEATURE_PV_OPS);
>> Not for Xen HVM guests.
> From what I can tell, HVM guests still use pv_time_ops and
> pv_mmu_ops.exit_mmap, right?
>
>>> +   apply_alternatives(__pv_alt_instructions, __pv_alt_instructions_end);
>>> +}
>>
>> This is a problem (at least for Xen PV guests):
>> apply_alternatives()->text_poke_early()->local_irq_save()->...'cli'->death.
> Ah, right.
>
>> It might be possible not to turn off/on the interrupts in this
>> particular case since the guest probably won't be able to handle an
>> interrupt at this point anyway.
> Yeah, that should work.  For Xen and for the other hypervisors, this is
> called well before irq init, so interrupts can't be handled yet anyway.

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?


-boris

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