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SubjectRe: [PATCH] x86, apic: Enable x2APIC physical when cpu < 256 native
On Tue, Jul 23, 2013 at 09:24:45PM -0700, Yinghai Lu wrote:
> On Thu, Jul 11, 2013 at 6:22 PM, Youquan Song <youquan.song@intel.com> wrote:
> > x2APIC extends APICID from 8 bits to 32 bits, but the device interrupt routed
> > from IOAPIC or delivered in MSI mode will keep 8 bits destination APICID.
> > In order to support x2APIC, the VT-d interrupt remapping is introduced to
> > translate the destination APICID to 32 bits in x2APIC mode and keep the device
> > compatible in this way.
> >
> > x2APIC support both logical and physical mode in destination mode.
> > In logical destination mode, the 32 bits Logical APICID has 2 sub-fields:
> > 16 bits cluster ID and 16 bits logical ID within the cluster and it is
> > required VT-d interrupt remapping in x2APIC cluster mode.
> > In physical destination mode, the 8 bits physical id is compatible with 32
> > bits physical id when CPU number < 256.
> > When interrupt remapping initialization fail on platform with CPU number < 256,
> > current kernel only enables x2APIC physical mode in virutalization environment,
> > while we also can enable x2APIC physcial mode in native kernel this situation,
> > and the device interrupt will use 8 bits destination APICID in physical mode
> > and be compatible with x2APIC physical when < 256 CPUs.
> >
> > So we can benefit from x2APIC vs xAPIC MMIO:
> > - x2APIC MSR read/write is faster than xAPIC mmio
> > - x2APIC only ICR write to deliver interrupt without polling ICR deliver
> > status bit and xAPIC need poll to read ICR deliver status bit.
> > - x2APIC 64 bits ICR access instead of xAPIC two 32 bits access.
> >
> > Signed-off-by: Youquan Song <youquan.song@intel.com>
> > ---
> > arch/x86/kernel/apic/apic.c | 7 ++-----
> > 1 files changed, 2 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> > index 904611b..51a065a 100644
> > --- a/arch/x86/kernel/apic/apic.c
> > +++ b/arch/x86/kernel/apic/apic.c
> > @@ -1603,11 +1603,8 @@ void __init enable_IR_x2apic(void)
> > goto skip_x2apic;
> >
> > if (ret < 0) {
> > - /* IR is required if there is APIC ID > 255 even when running
> > - * under KVM
> > - */
> > - if (max_physical_apicid > 255 ||
> > - !hypervisor_x2apic_available()) {
> > + /* IR is required if there is APIC ID > 255 */
> > + if (max_physical_apicid > 255) {
> > if (x2apic_preenabled)
> > disable_x2apic();
> > goto skip_x2apic;
>
> Those are kvm and xen related.
>
This change does not affect kvm or xen since they already enable x2apic
without IR. But back then when I added the apicid > 255 check I had to
add the hypervisor check too because I was told by Intel that x2apic
without IR is not architectural. Seeing that this patch comes from Intel
engineer it is possible that things changed since then.

--
Gleb.


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