Re: [PATCH v2 1/2] KVM: x86/xen: Initialize Xen timer only once

[Sean Christopherson]

On Fri, Jul 29, 2022, Coleman Dietsch wrote:
> Add a check for existing xen timers before initializing a new one.
> 
> Currently kvm_xen_init_timer() is called on every
> KVM_XEN_VCPU_ATTR_TYPE_TIMER, which is causing the following ODEBUG
> crash when vcpu->arch.xen.timer is already set.
> 
> ODEBUG: init active (active state 0)
> object type: hrtimer hint: xen_timer_callbac0
> RIP: 0010:debug_print_object+0x16e/0x250 lib/debugobjects.c:502
> Call Trace:
> __debug_object_init
> debug_hrtimer_init
> debug_init
> hrtimer_init
> kvm_xen_init_timer
> kvm_xen_vcpu_set_attr
> kvm_arch_vcpu_ioctl
> kvm_vcpu_ioctl
> vfs_ioctl
> 

Fixes: 536395260582 ("KVM: x86/xen: handle PV timers oneshot mode")
Cc: stable@xxxxxxxxxxxxxxx

> Link: https://syzkaller.appspot.com/bug?id=8234a9dfd3aafbf092cc5a7cd9842e3ebc45fc42
> Reported-by: syzbot+e54f930ed78eb0f85281@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Coleman Dietsch <dietschc@xxxxxxx>
> ---

One uber nit below, otherwise:

Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>

>  arch/x86/kvm/xen.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index 610beba35907..2dd0f72a62f2 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -713,7 +713,10 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
>  				break;
>  			}
>  			vcpu->arch.xen.timer_virq = data->u.timer.port;
> -			kvm_xen_init_timer(vcpu);
> +
> +			/* Check for existing timer */

I vote to omit the comment, "existing timer" is ambiguous, e.g. it could mean that
there's an existing _running_ timer as opposed to an existing initialized timer.
IMO the code is obvious enough on its own.

> +			if (!vcpu->arch.xen.timer.function)
> +				kvm_xen_init_timer(vcpu);
>  
>  			/* Restart the timer if it's set */
>  			if (data->u.timer.expires_ns)
> -- 
> 2.34.1
>