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

[Chegu Vinod <chegu_vinod@hp.com>]

On 10/13/2012 12:32 AM, Gleb Natapov wrote:
> 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.
> .
>


Retried with the above  patch and the guest is booting fine. (x2apic 
flag shows up in the guest's /proc/cpuinfo).

Was this a recent regression?

Thanks!
Vinod