Re: __schedule #DF splat

Gleb Natapov
2014-06-29T06:46:27Z

On Sat, Jun 28, 2014 at 01:44:31PM +0200, Borislav Petkov wrote:
> qemu-system-x86-20240 [006] ...1 9406.484134: kvm_page_fault: address 7fffb62ba318 error_code 2
> qemu-system-x86-20240 [006] ...1 9406.484136: kvm_inj_exception: #PF (0x2)a
>
> kvm injects the #PF into the guest.
>
> qemu-system-x86-20240 [006] d..2 9406.484136: kvm_entry: vcpu 1
> qemu-system-x86-20240 [006] d..2 9406.484137: kvm_exit: reason PF excp rip 0xffffffff8161130f info 2 7fffb62ba318
> qemu-system-x86-20240 [006] ...1 9406.484138: kvm_page_fault: address 7fffb62ba318 error_code 2
> qemu-system-x86-20240 [006] ...1 9406.484141: kvm_inj_exception: #DF (0x0)
>
> Second #PF at the same address and kvm injects the #DF.
>
> BUT(!), why?
>
> I probably am missing something but WTH are we pagefaulting at a
> user address in context_switch() while doing a lockdep call, i.e.
> spin_release? We're not touching any userspace gunk there AFAICT.
>
> Is this an async pagefault or so which kvm is doing so that the guest
> rip is actually pointing at the wrong place?
>
There is nothing in the trace that point to async pagefault as far as I see.

> Or something else I'm missing, most probably...
>
Strange indeed. Can you also enable kvmmmu tracing? You can also instrument
kvm_multiple_exception() to see which two exception are combined into #DF.

--
Gleb.