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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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.

?

(CC Jan as well)

Wei.

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