Re: [Qemu-devel] [PATCH V3] Guest stop notification

[Marcelo Tosatti <mtosatti@redhat.com>]

On Sat, Dec 03, 2011 at 10:06:56AM +0100, Jan Kiszka wrote:
> On 2011-12-02 22:27, Eric B Munson wrote:
> > On Fri, 02 Dec 2011, Jan Kiszka wrote:
> > 
> >> On 2011-12-02 20:19, 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@mgebm.net>
> >>> Cc: Avi Kivity <avi@redhat.com>
> >>> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> >>> Cc: Jan Kiszka <jan.kiszka@siemens.com>
> >>> Cc: ryanh@linux.vnet.ibm.com
> >>> Cc: aliguori@us.ibm.com
> >>> Cc: kvm@vger.kernel.org
> >>>
> >>> ---
> >>> Changes from V2:
> >>>  Move ioctl into hw/kvmclock.c so as other arches can use it as it is
> >>> implemented
> >>>
> >>> Changes from V1:
> >>>  Remove unnecessary encapsulating function
> >>>
> >>>  hw/kvmclock.c |   24 ++++++++++++++++++++++++
> >>>  1 files changed, 24 insertions(+), 0 deletions(-)
> >>>
> >>> diff --git a/hw/kvmclock.c b/hw/kvmclock.c
> >>> index 5388bc4..756839f 100644
> >>> --- a/hw/kvmclock.c
> >>> +++ b/hw/kvmclock.c
> >>> @@ -16,6 +16,7 @@
> >>>  #include "sysbus.h"
> >>>  #include "kvm.h"
> >>>  #include "kvmclock.h"
> >>> +#include "cpu-all.h"
> >>>  
> >>>  #include <linux/kvm.h>
> >>>  #include <linux/kvm_para.h>
> >>> @@ -69,11 +70,34 @@ static void kvmclock_vm_state_change(void *opaque, int running,
> >>>      }
> >>>  }
> >>>  
> >>> +static void kvmclock_vm_state_change_vcpu(void *opaque, int running,
> >>> +                                          RunState state)
> >>> +{
> >>> +    int ret;
> >>> +    CPUState *penv = first_cpu;
> >>> +
> >>> +    if (running) {
> >>> +	while (penv) {
> >>
> >> or: for (cpu = first_cpu; cpu != NULL; cpu = cpu->next_cpu) {
> >>
> > 
> > Functionally equivalent and I see both in the code, is there a standard?
> 
> Not really. I once tried to introduce an iterator macro, but it was
> refused. The above is just more compact.
> 
> But this is only a minor nit.
> 
> > 
> >>> +            ret = kvm_vcpu_ioctl(penv, KVM_GUEST_PAUSED, 0);
> >>> +            if (ret) {
> >>> +                if (ret != ENOSYS) {
> >>> +                    fprintf(stderr,
> >>> +                            "kvmclock_vm_state_change_vcpu: %s\n",
> >>> +                            strerror(-ret));
> >>> +                }
> >>> +                return;
> >>> +            }
> >>> +            penv = (CPUState *)penv->next_cpu;
> >>
> >> Unneeded cast.
> >>
> > 
> > Also following an example seen elsewhere.
> 
> Generally, we try to avoid those pointless casts.
> 
> > 
> >>> +        }
> >>> +    }
> >>> +}
> >>> +
> >>
> >> Again: please use checkpatch.pl.
> >>
> > 
> > Sorry, tough to get used to hitting space bar that many times...
> > 
> >>>  static int kvmclock_init(SysBusDevice *dev)
> >>>  {
> >>>      KVMClockState *s = FROM_SYSBUS(KVMClockState, dev);
> >>>  
> >>>      qemu_add_vm_change_state_handler(kvmclock_vm_state_change, s);
> >>> +    qemu_add_vm_change_state_handler(kvmclock_vm_state_change_vcpu, NULL);
> >>>      return 0;
> >>>  }
> >>>  
> >>
> >> Why not extend the existing handler?
> > 
> > Because the new handler doesn't touch the KVMClockState object.  If this is
> > preferred, I have no objection.
> 
> The separate registration looks strange to me. And the fact that you
> don't need to object doesn't justify a callback of its own.
> 
> > 
> >>
> >> I still wonder if the IOCTL interface is actually kvmclock specific. But
> >> Marcello asked for this, and we could still change it when some arch
> >> comes around that provides it independent of kvmclock.
> > 
> > The flag itself is stored in the pvclock_vcpu_time_info structure, and anything
> > else that touches that structure uses ioctls.
> 
> That's the host-guest interface. But I'm talking about the kvm-qemu
> interface here which has no relation to how the "was paused" information
> is transferred to the guest.
> 
> Jan

It is one simple, rarely used command. I don't see why another interface
such as kvm_run would be beneficial for this case.