On 02/12/2016 20:44, Radim Krčmář wrote:
> LAPIC after reset is in xAPIC mode, which poses a problem for hotplug of
> VCPUs with high APIC ID, because reset VCPU is waiting for INIT/SIPI,
> but there is no way to uniquely address it using xAPIC.
>
> From many possible options, we chose the one that also works on real
> hardware: accepting interrupts addressed to LAPIC's x2APIC ID even in
> xAPIC mode.
>
> KVM intentionally differs from real hardware, because real hardware
> (Knights Landing) does just "x2apic_id & 0xff" to decide whether to
> accept the interrupt in xAPIC mode and it can deliver one interrupt to
> more than one physical destination, e.g. 0x123 to 0x123 and 0x23.
>
> Add a capability to let userspace know that we do something now.
I wouldn't even bother with the capability. It's a bit borderline for
stable, but I think it's okay. We can always Cc and let the maintainer
reject it.
Paolo
> Signed-off-by: Radim Krčmář <rkrcmar@xxxxxxxxxx>
> ---
> arch/x86/kvm/lapic.c | 13 +++++++++++--
> arch/x86/kvm/x86.c | 1 +
> include/uapi/linux/kvm.h | 1 +
> 3 files changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 42edf1ea2909..b85985985ac8 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -199,10 +199,15 @@ static void recalculate_apic_map(struct kvm *kvm)
> xapic_id = kvm_xapic_id(apic);
> x2apic_id = kvm_x2apic_id(apic);
>
> - if (apic_x2apic_mode(apic) &&
> + /* Hotplug hack: see kvm_apic_match_physical_addr(), ... */
> + if ((apic_x2apic_mode(apic) || x2apic_id > 0xff) &&
> x2apic_id <= new->max_apic_id)
> new->phys_map[x2apic_id] = apic;
> - else if (!apic_x2apic_mode(apic))
> + /*
> + * ... xAPIC ID of VCPUs with APIC ID > 0xff will wrap-around,
> + * prevent them from masking VCPUs with APIC ID <= 0xff.
> + */
> + if (!apic_x2apic_mode(apic) && !new->phys_map[xapic_id])
> new->phys_map[xapic_id] = apic;
>
> ldr = kvm_lapic_get_reg(apic, APIC_LDR);
> @@ -612,6 +617,10 @@ static bool kvm_apic_match_physical_addr(struct kvm_lapic *apic, u32 mda)
> if (apic_x2apic_mode(apic))
> return mda == kvm_x2apic_id(apic);
>
> + /* Hotplug hack: LAPIC in xAPIC mode also accepts x2APIC. */
> + if (kvm_x2apic_id(apic) > 0xff && mda == kvm_x2apic_id(apic))
> + return true;
> +
> return mda == kvm_xapic_id(apic);
> }
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 7770d77c828d..945e8eeb4eb1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2618,6 +2618,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_DISABLE_QUIRKS:
> case KVM_CAP_SET_BOOT_CPU_ID:
> case KVM_CAP_SPLIT_IRQCHIP:
> + case KVM_CAP_X86_ALWAYS_ACCEPT_X2APIC_DEST:
> #ifdef CONFIG_KVM_DEVICE_ASSIGNMENT
> case KVM_CAP_ASSIGN_DEV_IRQ:
> case KVM_CAP_PCI_2_3:
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index e9f5ceffd741..f25efa375255 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -871,6 +871,7 @@ struct kvm_ppc_smmu_info {
> #define KVM_CAP_S390_USER_INSTR0 130
> #define KVM_CAP_MSI_DEVID 131
> #define KVM_CAP_PPC_HTM 132
> +#define KVM_CAP_X86_ALWAYS_ACCEPT_X2APIC_DEST 133
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
>
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