Re: [Qemu-devel] Unable to enable +x2apic for the guest cpus...

[Gleb Natapov]

On Fri, Oct 12, 2012 at 07:38:42PM -0700, Chegu Vinod wrote:
> Hello,
> 
> I am using a very recent upstream version of qemu.git along with
> kvm.git kernels (in the host and guest).
>  [Guest kernel had been compiled with CONFIG_X86_X2APIC and
> CONFIG_IRQ_REMAP both set]
> 
> When I attempt to start a guest with +x2apic flag (pl. see the qemu
> cmd line below) I end up with a hang of the qemu and
> a kernel BUG at /arch/x86/kvm/lapic.c:159 !    Pl. see the attached
> screen shot of the console for additional info.
> 
> I am able to boot the same guest without the +x2apic flag in the
> qemu cmd line.
> 
> Not sure if this an issue (or) if I have something incorrectly
> specified in the qemu cmd line ? If its the latter...pl. advise the
> correct usage
> for enabling x2apic for the guest cpus.. for the upstream bits.
> 
This is the bug in how ldr in x2apic mode is calculated.

Try the following patch:

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index c6e6b72..43e9fad 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1311,7 +1311,7 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
        vcpu->arch.apic_base = value;
        if (apic_x2apic_mode(apic)) {
                u32 id = kvm_apic_id(apic);
-               u32 ldr = ((id & ~0xf) << 16) | (1 << (id & 0xf));
+               u32 ldr = ((id >> 4) << 16) | (1 << (id & 0xf));
                kvm_apic_set_ldr(apic, ldr);
        }
        apic->base_address = apic->vcpu->arch.apic_base &
--
                        Gleb.