On Thu, Jul 11, 2013 at 6:22 PM, Youquan Song <youquan.s...@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.s...@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. Add more Cc. Yinghai -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/