linux-kernel - Re: [PATCH] Guest stop notification

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Date:	Thu, 1 Dec 2011 12:19:38 -0500
From:	Eric B Munson <emunson@...bm.net>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	qemu-devel@...gnu.org, ryanh@...ux.vnet.ibm.com,
	aliguori@...ibm.com, kvm@...r.kernel.org, mtosatti@...hat.com,
	linux-kernel@...r.kernel.org, avi@...hat.com
Subject: Re: [PATCH] Guest stop notification

On Thu, 01 Dec 2011, Jan Kiszka wrote:

> On 2011-11-29 22:36, Eric B Munson wrote:
> > Often when a guest is stopped from the qemu console, it will report spurious
> > soft lockup warnings on resume.  There are kernel patches being discussed that
> > will give the host the ability to tell the guest that it is being stopped and
> > should ignore the soft lockup warning that generates.
> > 
> > Signed-off-by: Eric B Munson <emunson@...bm.net>
> > Cc: ryanh@...ux.vnet.ibm.com
> > Cc: aliguori@...ibm.com
> > Cc: mtosatti@...hat.com
> > Cc: avi@...hat.com
> > Cc: kvm@...r.kernel.org
> > Cc: linux-kernel@...r.kernel.org
> > ---
> >  target-i386/kvm.c |    6 ++++++
> >  1 files changed, 6 insertions(+), 0 deletions(-)
> > 
> > diff --git a/target-i386/kvm.c b/target-i386/kvm.c
> > index 5bfc21f..defd364 100644
> > --- a/target-i386/kvm.c
> > +++ b/target-i386/kvm.c
> > @@ -336,12 +336,18 @@ static int kvm_inject_mce_oldstyle(CPUState *env)
> >      return 0;
> >  }
> >  
> > +static void kvm_put_guest_paused(CPUState *penv)
> > +{
> > +    kvm_vcpu_ioctl(penv, KVM_GUEST_PAUSED, 0);
> > +}
> 
> I see no need in encapsulating this in a separate function.
> 

The encapsulated function was from a previous idea, I will remove it for V2.

> > +
> >  static void cpu_update_state(void *opaque, int running, RunState state)
> >  {
> >      CPUState *env = opaque;
> >  
> >      if (running) {
> >          env->tsc_valid = false;
> > +	kvm_put_guest_paused(env);
> 
> checkpatch.pl would have asked you to remove this tab.

Will change to spaces for V2.

> 
> More general:
> 
> Why is this x86-only? If the kernel interface is x86-only, what prevents
> making it generic right from the beginning?
> 
> Why do we need a new IOCTL for this? Was there no space left in the
> kvm_run structure e.g. to pass this flag down on next vcpu execution? No
> big deal, just wondering.

Thanks for your review/feedback.

When I started looking into this problem, the ioctl was the first suggestion I
got for how to communicate from qemu to guest kernel.  I don't see a technical
reason that this could not be added to the kvm_run structure in one of the
bytes currently used as padding.  I would prefer to keep the ioctl because I
have the corresponding kernel patches out to work with this, however, if there
is a strong preference for using kvm_run, I can rework both sets.

Eric


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