linux-kernel - Re: [PATCH 3/5 V5] Add ioctl for KVMCLOCK_GUEST

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Message-ID: <20111208152520.GC6301@mgebm.net>
Date:	Thu, 8 Dec 2011 10:25:20 -0500
From:	Eric B Munson <emunson@...bm.net>
To:	Avi Kivity <avi@...hat.com>
Cc:	mingo@...hat.com, hpa@...or.com, arnd@...db.de,
	ryanh@...ux.vnet.ibm.com, aliguori@...ibm.com, mtosatti@...hat.com,
	jeremy.fitzhardinge@...rix.com, levinsasha928@...il.com,
	Jan Kiszka <jan.kiszka@...mens.com>, kvm@...r.kernel.org,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5 V5] Add ioctl for KVMCLOCK_GUEST_STOPPED

On Wed, 07 Dec 2011, Avi Kivity wrote:

> On 12/05/2011 10:19 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.
> >
> > @@ -3295,6 +3295,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> >  
> >  		goto out;
> >  	}
> > +	case KVMCLOCK_GUEST_PAUSED: {
> > +		r = kvm_set_guest_paused(vcpu);
> > +		break;
> > +	}
> >  	default:
> >  		r = -EINVAL;
> >  	}
> >
> 
> You could also do this purely from userspace by reading the kvmclock msr
> and updating it.  However, it's better to do this in the kernel to avoid
> distributing responsibility for kvmclock across too many cooks.
> 
> -- 
> error compiling committee.c: too many arguments to function
> 

As stated in the cover letter, per Marcelo's suggestion I already tried this
method and it resulted in the guest wall clock skewing by the amount of time
the guest was suspended.  This places a new burden on the guest to be running
ntpd or equivslent to keep the wall clock correct.

Eric

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