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    SubjectRe: PANIC: double fault, error_code: 0x0 in 4.0.0-rc3-2, kvm related?
    On 03/23/2015 06:18 PM, Takashi Iwai wrote:
    > At Mon, 23 Mar 2015 17:07:15 +0100, Denys Vlasenko wrote:
    >>>> I pulled tip tree on top of 4.0-rc5, built with your patch and now
    >>>> succeeded to get a better message:
    >>>>
    >>>> kvm: zapping shadow pages for mmio generation wraparound
    >>>> kvm [5126]: vcpu0 disabled perfctr wrmsr: 0xc1 data 0xffff
    >>>> Exception on user stack 00007ffd22c23ef0: RSP: 0018:00007ffd22c23f28 EFLAGS: 00010006
    >>>> RIP: 0010:[<ffffffff8162681d>] [<ffffffff8162681d>] netlink_attachskb+0x1d/0x1d0
    >>>> PANIC: double fault, error_code: 0x0
    >>>> CPU: 1 PID: 10819 Comm: cc1 Tainted: G W 4.0.0-rc5-debug1+ #2
    >>>> Hardware name: Dell Inc. OptiPlex 9010/0M9KCM, BIOS A12 01/10/2013
    >>>> task: ffff8800d1b34b10 ti: ffff8800d1b30000 task.ti: ffff8800d1b30000
    >>>> RIP: 0010:[<ffffffff8162681d>] [<ffffffff8162681d>] netlink_attachskb+0x1d/0x1d0
    >>>> RSP: 0018:00007ffd22c23f28 EFLAGS: 00010006
    >>>> RAX: 0000000000000000 RBX: 0000000000000005 RCX: 00000000c0000101
    >>>> RDX: 0000000000000000 RSI: 0000000000000001 RDI: 00007ffd22c23ef0

    >> FYI: the disassembly of netlink_attachskb (from "Code:" line) is:
    >>
    >> 0: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
    >> 5: 55 push %rbp
    >> 6: 48 89 e5 mov %rsp,%rbp
    >> 9: 41 56 push %r14
    >> b: 41 55 push %r13
    >> d: 49 89 d5 mov %rdx,%r13
    >> 10: 41 54 push %r12
    >> 12: 49 89 f4 mov %rsi,%r12
    >> 15: 53 push %rbx
    >> 16: 48 89 fb mov %rdi,%rbx
    >> 19: 48 83 ec 30 sub $0x30,%rsp
    >> 1d: 8b 87 68 01 00 00 mov 0x168(%rdi),%eax
    >> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    >> 23: 39 87 9c 01 00 00 cmp %eax,0x19c(%rdi)
    >> 29: 7c 25 jl 50 <_start+0x50>
    >> 2b: 48 8b 87 88 04 00 00 mov 0x488(%rdi),%rax
    >>
    >> The ^^^^^ instruction is the one which faults. Since you said it
    >> consistently happens here, this should be a page fault, not an external
    >> hardware interrupt.
    >>
    >> The code corresponds to the comparison in if():
    >>
    >> int netlink_attachskb(struct sock *sk, struct sk_buff *skb,
    >> long *timeo, struct sock *ssk)
    >> {
    >> struct netlink_sock *nlk;
    >>
    >> nlk = nlk_sk(sk);
    >>
    >> if ((atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf ||

    >>> - Another piece is that the bug happens only when a KVM is running.
    >>> The kernel ran without problem over days with similar tasks
    >>> (compiling kernel, etc) when no KVM was used.
    >>
    >> Conceivably virtualization support in CPUs can have nasty erratas.
    >> However, you and other reporter have different CPUs - yours
    >> is Ivy Bridge, his CPU is a Penryn.
    >>
    >> I don't see the path how KVM helps to trigger this.
    >>
    >>> - And now I get the trace as above, pointing netlink_attachskb().
    >>>
    >>> I have a difficulty to imagine how all these pieces fit into a single
    >>> picture. Is something already screwed up before that?
    >>
    >> Well, a tiny bit more info will be seen if you'd change %rdi
    >> to, say, %r15 in these two lines in my patch:
    >>
    >> /* Save bogus RSP value */
    >> movq %rsp,%rdi
    >> ...
    >> push %rdi /* pt_regs->sp */
    >>
    >> Then original %rdi will be visible in the crash message.
    >
    > OK, here we go.
    >
    > kvm: zapping shadow pages for mmio generation wraparound
    > kvm [5490]: vcpu0 disabled perfctr wrmsr: 0xc1 data 0xffff
    > Exception on user stack 00007fff1d7e5ec0: RSP: 0018:00007fff1d7e5ef8 EFLAGS: 00010002
    > RIP: 0010:[<ffffffff8162681d>] [<ffffffff8162681d>] netlink_attachskb+0x1d/0x1d0
    > PANIC: double fault, error_code: 0x0
    > CPU: 5 PID: 14285 Comm: fixdep Tainted: G W 4.0.0-rc5-debug1+ #3
    > Hardware name: Dell Inc. OptiPlex 9010/0M9KCM, BIOS A12 01/10/2013
    > task: ffff88020ba1c690 ti: ffff880206ba4000 task.ti: ffff880206ba4000
    > RIP: 0010:[<ffffffff8162681d>] [<ffffffff8162681d>] netlink_attachskb+0x1d/0x1d0
    > RSP: 0018:00007fff1d7e5ef8 EFLAGS: 00010002
    > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00000000c0000101
    > RDX: 0000000000000000 RSI: 0000000000001ebb RDI: 0000000000000000

    Thanks for your testing. So the %rdi was NULL... not very informative.

    Notice that your every crash is preceded by

    kvm: zapping shadow pages for mmio generation wraparound
    kvm [5490]: vcpu0 disabled perfctr wrmsr: 0xc1 data 0xffff

    This hints that kvm _is_ somehow responsible.
    I'm no expert on kvm, I need to take a look around that code...


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