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SubjectRe: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure
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.)

--
Josh

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