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

[Gleb Natapov]

On Sat, Oct 13, 2012 at 05:22:40AM -0700, Chegu Vinod wrote:
> 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?
> 
The bug was there for as long as x2apic support, but it was benign till
the resent apic optimization. We should treat it as a regression.

--
                        Gleb.