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

[Coleman Dietsch]

On Fri, Aug 05, 2022 at 06:37:13PM +0000, Sean Christopherson wrote:
> 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>
> 

Got it, thanks Sean.

> >  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.
> 

This makes perfect sense. I will ommit the comment in the next patch.

> > +			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
> >