Re: [Xen-devel] [PATCH v3 2/4] x86/apic: force phys mode if interrupt remapping is disabled

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

On Thu, Dec 05, 2019 at 10:32:34AM +0100, Jan Beulich wrote:
> On 04.12.2019 17:20, 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>
> 
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> albeit ...
> 
> > --- a/xen/arch/x86/genapic/x2apic.c
> > +++ b/xen/arch/x86/genapic/x2apic.c
> > @@ -226,7 +226,23 @@ boolean_param("x2apic_phys", x2apic_phys);
> >  const struct genapic *__init apic_x2apic_probe(void)
> >  {
> >      if ( x2apic_phys < 0 )
> > -        x2apic_phys = !!(acpi_gbl_FADT.flags & ACPI_FADT_APIC_PHYSICAL);
> > +    {
> > +        if ( !iommu_intremap )
> > +            /*
> > +             * Force physical mode if there's no interrupt remapping 
> > support:
> > +             * the ID in clustered mode requires a 32 bit destination 
> > field due
> > +             * to the usage of the high 16 bits to store the cluster ID.
> > +             */
> > +            x2apic_phys = true;
> > +        else
> > +            x2apic_phys = !!(acpi_gbl_FADT.flags & 
> > ACPI_FADT_APIC_PHYSICAL);
> 
> ... I wonder why you didn't make this
> 
>         x2apic_phys = !iommu_intremap || (acpi_gbl_FADT.flags & 
> ACPI_FADT_APIC_PHYSICAL);
> 
> (not the least because of allowing to drop the somewhat ugly !!).

Feel free to do it at commit (and reindent the comment), or else I can
resend a new version if that's too intrusive.

Thanks, Roger.

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