linux-kernel - Re: [PATCH 3/4 V10] Add ioctl for KVMCLOCK_GUEST

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Message-ID: <20120130153326.GB6875@mgebm.net>
Date:	Mon, 30 Jan 2012 10:33:26 -0500
From:	Eric B Munson <emunson@...bm.net>
To:	Avi Kivity <avi@...hat.com>
Cc:	mingo@...hat.com, hpa@...or.com, ryanh@...ux.vnet.ibm.com,
	aliguori@...ibm.com, mtosatti@...hat.com,
	jeremy.fitzhardinge@...rix.com, kvm@...r.kernel.org,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4 V10] Add ioctl for KVMCLOCK_GUEST_STOPPED

On Mon, 30 Jan 2012, Avi Kivity wrote:

> On 01/30/2012 05:07 PM, Avi Kivity wrote:
> > > +Parameters: None
> > > +Returns: 0 on success, -1 on error
> > > +
> > > +This signals to the host kernel that the specified guest is being paused by
> > > +userspace.  The host will set a flag in the pvclock structure that is checked
> > > +from the soft lockup watchdog.  This ioctl can be called during pause or
> > > +unpause.
> > > +
> > >  5. The kvm_run structure
> > >  
> > >  
> > > +/*
> > > + * kvm_set_guest_paused() indicates to the guest kernel that it has been
> > > + * stopped by the hypervisor.  This function will be called from the host only.
> > > + */
> > > +static int kvm_set_guest_paused(struct kvm *kvm)
> > > +{
> > > +	struct kvm_vcpu *vcpu;
> > > +	struct pvclock_vcpu_time_info *src;
> > > +	int i;
> > > +
> > > +	kvm_for_each_vcpu(i, vcpu, kvm) {
> > > +		if (!vcpu->arch.time_page)
> > > +			continue;
> > > +		src = &vcpu->arch.hv_clock;
> > > +		src->flags |= PVCLOCK_GUEST_STOPPED;
> >
> > This looks racy.  The vcpu can remove its kvmclock concurrently with
> > this access, and src will be NULL.
> >
> > Can you point me to the discussion that moved this to be a vm ioctl?  In
> > general vm ioctls that do things for all vcpus are racy, like here. 
> > You're accessing variables that are protected by the vcpu mutex, and not
> > taking the mutex (nor can you, since it is held while the guest is
> > running, unlike most kernel mutexes).
> >
> 
> Note, the standard way to fix this race is to
> kvm_make_request(KVM_REQ_KVMCLOCK_STOP) and kvm_vcpu_kick(vcpu), and do
> the update in vcpu_enter_guest().  But I think this is needless
> complexity and want to understand what motivated the move to a vm ioctl.
> 

I will hold off on fixing this race until we settle the vm or vcpu ioctl
question.

Eric

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