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

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Date:	Mon, 30 Jan 2012 10:32:44 -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 01/17/2012 08:40 PM, Eric B Munson wrote:
> > Now that we have a flag that will tell the guest it was suspended, create an
> > interface for that communication using a KVM ioctl.
> >
> >
> > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > index e1d94bf..1931e5c 100644
> > --- a/Documentation/virtual/kvm/api.txt
> > +++ b/Documentation/virtual/kvm/api.txt
> > @@ -1491,6 +1491,19 @@ following algorithm:
> >  Some guests configure the LINT1 NMI input to cause a panic, aiding in
> >  debugging.
> >  
> > +4.65 KVMCLOCK_GUEST_PAUSED
> > +
> > +Capability: KVM_CAP_GUEST_PAUSED
> > +Architechtures: Any that implement pvclocks (currently x86 only)
> > +Type: vcpu ioctl
> 
> vm ioctl.
> 
> > +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).
> 

Jan Kiszka suggested that becuase there isn't a use case for notifying
individual vcpus (can vcpu's be paused individually?) that it makes more sense
to have a vm ioctl.

http://thread.gmane.org/gmane.comp.emulators.qemu/131624

If the per vcpu ioctl is the right choice I can resend those patches.

Eric

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