[Qemu-devel] heads up: guest reset after migration using --enable-kvm

[Juan Quintela]

Hi

As I spent too much time last week tracking this bug, just to let you
know.  If you run qemu --enable-kvm, and you do migration or loadvm from
the command line, guest just loads correctly and then reset with a
triple fault.

Problem happens if you are using a 2.6.30.x kernel, this is going to be
reverted  on the next stable kernel.  2.6.31 suffers the same problem.

qemu-kvm.git works here, problem is with qemu --enable-kvm.

If you do loadvm from the command monitor after having run another
guests, things work as expected.

Later, Juan.

> --- Begin Message ---
>
>
>
> Subject: 
>
> [PATCH 2.6.31 STABLE] Revert "KVM: x86: check for cr3 validity in	ioctl_set_sregs"
>
>
>
>
> Date: 
>
> Mon, 28 Sep 2009 15:09:55 -0300
>
>
>
>
> User-agent: 
>
> Mutt/1.5.19 (2009-01-05)
>
>
>
>
> This reverts commit 6c20e1442bb1c62914bb85b7f4a38973d2a423ba.
>
> To my understanding, it became obsolete with the advent of the more
> robust check in mmu_alloc_roots (89da4ff17f). Moreover, it prevents
> the conceptually safe pattern
>
>  1. set sregs
>  2. register mem-slots
>  3. run vcpu
>
> by setting a sticky triple fault during step 1.
>
> Signed-off-by: Jan Kiszka <address@hidden>
> Signed-off-by: Avi Kivity <address@hidden>
> (cherry picked from commit dc7e795e3dd2a763e5ceaa1615f307e808cf3932)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3d36045..2e0b363 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4118,13 +4118,7 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
>  
>         vcpu->arch.cr2 = sregs->cr2;
>         mmu_reset_needed |= vcpu->arch.cr3 != sregs->cr3;
> -
> -       down_read(&vcpu->kvm->slots_lock);
> -       if (gfn_to_memslot(vcpu->kvm, sregs->cr3 >> PAGE_SHIFT))
> -               vcpu->arch.cr3 = sregs->cr3;
> -       else
> -               set_bit(KVM_REQ_TRIPLE_FAULT, &vcpu->requests);
> -       up_read(&vcpu->kvm->slots_lock);
> +       vcpu->arch.cr3 = sregs->cr3;
>  
>         kvm_set_cr8(vcpu, sregs->cr8);
>  
>
> --- End Message ---