Re: [Xen-devel] [PATCH RFC V4 4/5] kvm : pv-ticketlocks support for linux guests running on KVM hypervisor

[Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>]

On 01/24/2012 07:38 PM, Avi Kivity wrote:
> On 01/18/2012 11:52 PM, Jeremy Fitzhardinge wrote:
> > On 01/19/2012 12:54 AM, Srivatsa Vaddagiri wrote:
> > > > That logic relies on the "kick" being level triggered, so that "kick"
> > > > before "block" will cause the block to fall out immediately.  If you're
> > > > using "hlt" as the block and it has the usual edge-triggered behaviour,
> > > > what stops a "kick-before-hlt" from losing the kick?
> > > Hmm ..'hlt' should result in a check for kick request (in hypervisor
> > > context) before vcpu is put to sleep. IOW vcpu1 that is attempting to kick vcpu0
> > > will set a 'somebody_tried_kicking_vcpu0' flag, which hypervisor should check
> > > before it puts vcpu0 to sleep because of trapped 'hlt' instruction.
> > >
> > > Won't that trap the 'kick-before-hlt' case? What am I missing here?
> > Nothing, that sounds fine.  It wasn't clear to me that your kick
> > operation left persistent state, and so has a level-triggered effect on hlt.
> btw, this persistent state needs to be saved/restored for live
> migration.  Best to put it into some MSR.
I did not quite get it. Did you mean, add a new MSR to msrs_to_save[],
and may be retain only the kicked/pv_unhalt flag (persistent state) and 
get rid of PVLOCK_KICK vcpu->request?

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