Re: [Xen-devel] [PATCH v2 2/4] x86/apic: force phys mode if interrupt remapping is disabled

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

On Tue, Dec 03, 2019 at 04:14:46PM +0100, Jan Beulich wrote:
> On 29.11.2019 12:38, Roger Pau Monné  wrote:
> > On Fri, Nov 29, 2019 at 12:28:49PM +0100, Roger Pau Monne wrote:
> >> Cluster mode can only be used with interrupt remapping support, since
> >> the top 16bits of the APIC ID are filled with the cluster ID, and
> >> hence on systems where the physical ID is still smaller than 255 the
> >> cluster ID is not. Force x2APIC to use physical mode if there's no
> >> interrupt remapping support.
> >>
> >> Note that this requires a further patch in order to enable x2APIC
> >> without interrupt remapping support.
> >>
> >> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > 
> > This is missing a command line doc update and the logic below ignores
> > a user-set x2apic_phys value.
> 
> So what would the behavior be in your opinion when the user
> has requested cluster mode? I can't see you do much other
> than panic()-ing, perhaps it's better to override the request
> (as you already do)?

I think panic'ing is fine, a user shouldn't be setting x2apic_phys
unless they know what are doing, and then Xen changing it on the back
of the user also doesn't seem fine.

A panic explaining that x2apic_phys=false is not supported and that
the box can only be booted with x2apic phys mode should be fine IMO.

Thanks, Roger.

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