Re: [Xen-devel] [PATCH] xl: remove apic option for PVH guests

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Mar 05, 2018 at 08:43:48AM -0600, Doug Goldstein wrote:
> On 3/2/18 5:29 AM, Jan Beulich wrote:
> >>>> On 02.03.18 at 12:09, <wei.liu2@xxxxxxxxxx> wrote:
> >> On Thu, Mar 01, 2018 at 05:01:55PM +0000, Roger Pau Monné wrote:
> >>> On Thu, Mar 01, 2018 at 04:01:23PM +0000, Wei Liu wrote:
> >>>> On Thu, Mar 01, 2018 at 03:57:18PM +0000, Andrew Cooper wrote:
> >>>>> On 01/03/18 12:22, Wei Liu wrote:
> >>>>>> On Wed, Feb 28, 2018 at 10:20:53AM +0000, Roger Pau Monne wrote:
> >>>>>>> XSA-256 forces the local APIC to always be enabled for PVH guests, so
> >>>>>>> ignore any apic option for PVH guests. Update the documentation
> >>>>>>> accordingly.
> >>>>>> I think how I will approach this is to dictate that PVH always has 
> >>>>>> LAPIC
> >>>>>> in our in-tree document, then use that as the justification for this
> >>>>>> change. That's the consensus from 2 years ago, right?
> >>>>>>
> >>>>>> Or we're just working around the limitation in our code base, and users
> >>>>>> may demand a no-LAPIC PVH guest just because...
> >>>>>
> >>>>> Currently, Xen enforces that HVM guests have an LAPIC.  This is because
> >>>>> making the non-LAPIC case function correctly/safely devolved into a
> >>>>> massive rats nest and I stopped trying to fix it after 2 days of trying.
> >>>>>
> >>>>> At the moment, it would be wise to discuss whether the non-LAPIC case is
> >>>>> actually sensible.  I personally see no value in keeping it.
> >>>>>
> >>>>
> >>>> +1
> >>>>
> >>>>> If someone can come up with a convincing usecase for keeping it, then
> >>>>> ok, but the barrier for this is increasing all the time, especially now
> >>>>> that hardware acceleration and posted interrupts means that a
> >>>>> pipeline-virtualised APIC is faster and more efficient than any of our
> >>>>> event channel mechanisms.
> >>>>
> >>>> +1
> >>>
> >>> I've looked at the in-tree pvh document and it just refers to the local
> >>> APIC in this sentence:
> >>>
> >>> "AP startup can be performed using hypercalls or the local APIC if 
> >>> present."
> >>>
> >>> I guess the trailing "if present" could be removed, but it's not
> >>> colliding with this patch.
> >>>
> >>> I'm happy with rebasing this patch and applying the above change, is
> >>> there any other document that should be changed?
> >>
> >> Can we make it more explicit. Like
> >>
> >>   VCPUs for PVH must have local APIC and it can't be disabled.
> >>
> >> ?
> > 
> > To be honest I liker Roger's suggestion better. And yet better
> > would imo be if we left that sentence alone, unless we really mean
> > to close that road for anyone wanting to take on making APIC-
> > less guests work securely.
> > 
> > Jan
> 
> I believe that's exactly what Andrew proposed in
> https://lists.xenproject.org/archives/html/xen-devel/2018-03/msg00089.html
> removing the wording doesn't exclude someone from adding it later but it
> does make it clear that its not available today.

I'm kind of lost regarding whether we reached consensus or not. Is the
current patch suitable, or should I change some of the wording?

Thanks, Roger.

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