Re: [Xen-devel] [PATCH v6 11/13] xen: support the Null scheduler

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 06/29/2018 07:38 PM, Stefano Stabellini wrote:
> On Thu, 28 Jun 2018, Roger Pau Monné wrote:
> > On Thu, Jun 28, 2018 at 09:27:08AM +0200, Dario Faggioli wrote:
> > > On Thu, 2018-06-14 at 13:20 -0700, Stefano Stabellini wrote:
> > > > On Thu, 14 Jun 2018, Andrew Cooper wrote:
> > > > > On 14/06/18 14:40, Jan Beulich wrote:
> > > > > I don't think its reasonable to alter the support status with this
> > > > issue
> > > > > outstanding.
> > > > I completely missed this report, probably because I haven't paid
> > > > attention to PV-shim. Do you have any more information about this?
> > > > The
> > > > report is a bit vague. If I can't repro it, I can't fix it.
> > > >
> > > > Couldn't it be that is normal because after a while you ran out of
> > > > pcpus?
> > > >
> > > > Dario, do you have any opinion on this?
> > > The issue that I know of is that the null scheduler does not properly
> > > support CPU hotplug/hotunplug.
> > >
> > > This is an issue on, let's say, baremetal, if you use null, and try to
> > > do CPU hotplug/hotunplug. When trying to use null as the scheduler of
> > > the shim, we run into that same issue, even if not specifically doing
> > > CPU hotplug/hotunplug (because the shim use the same path for CPU
> > > bringup, IIRC).
> > The shim uses CPU hotplug/unplug when the guest brings up/down a
> > vCPU using the VCPUOP_{up/down} hypercall.
> >
> > The best description of the issue I could find is:
> >
> > https://lists.xenproject.org/archives/html/xen-devel/2018-01/msg01085.html
> OK, thanks for the explanation. We don't support CPU hotplug on ARM, so
> we could mark the NULL scheduler as supported on the ARM architecture
> today? Once you implement CPU hotplug support in NULL, we could mark it
> as supported on x86 too.
Well, Mirela paved the way to support CPU hotplug (should be merged 
soon). She is looking at suspend/resume which is IHMO an extension of 
hotplug case. So are you sure this could never happen on Arm?
Also, supporting the hypercall would basically be ~20 lines. So I don't 
feel it is right to mark NULL scheduler supported on Arm until that is 
fixed in a way or another.
Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel