Re: [Xen-devel] Ping: [PATCH] VMX: sync CPU state upon vCPU destruction

[Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>]

On Tue, 2017-11-21 at 08:29 -0700, Jan Beulich wrote:
> > > > On 21.11.17 at 15:07, <igor.druzhinin@xxxxxxxxxx> wrote:
> > 
> > On 21/11/17 13:22, Jan Beulich wrote:
> > > > > > On 09.11.17 at 15:49, <JBeulich@xxxxxxxx> wrote:
> > > > 
> > > > See the code comment being added for why we need this.
> > > > 
> > > > Reported-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> > > > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > > 
> > > I realize we aren't settled yet on where to put the sync call. The
> > > discussion appears to have stalled, though. Just to recap,
> > > alternatives to the placement below are
> > > - at the top of complete_domain_destroy(), being the specific
> > >   RCU callback exhibiting the problem (others are unlikely to
> > >   touch guest state)
> > > - in rcu_do_batch(), paralleling the similar call from
> > >   do_tasklet_work()
> > 
> > rcu_do_batch() sounds better to me. As I said before I think that the
> > problem is general for the hypervisor (not for VMX only) and might
> > appear in other places as well.
> 
> The question here is: In what other cases do we expect an RCU
> callback to possibly touch guest state? I think the common use is
> to merely free some memory in a delayed fashion.
> 
> > Those choices that you outlined appear to be different in terms whether
> > we solve the general problem and probably have some minor performance
> > impact or we solve the ad-hoc problem but make the system more
> > entangled. Here I'm more inclined to the first choice because this
> > particular scenario the performance impact should be negligible.
> 
> For the problem at hand there's no question about a
> performance effect. The question is whether doing this for _other_
> RCU callbacks would introduce a performance drop in certain cases.

So what are performance implications of my original suggestion of
removing !v->is_running check from vmx_ctxt_switch_from() ?
From what I can see:

1. Another field in struct vcpu will be checked instead (vmcs_pa)
2. Additionally this_cpu(current_vmcs) will be loaded, which shouldn't
   be terrible, given how heavy a context switch already is.

-- 
Thanks,
Sergey
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