Re: [patch 4/4] QEMU/KVM: dont read any bits from userspace APIC emulation if its done in-kernel

[Marcelo Tosatti <mtosatti@redhat.com>]

On Sun, Apr 06, 2008 at 12:18:20PM +0300, Avi Kivity wrote:
> Marcelo Tosatti wrote:
> >Fixes loadvm/savem on SMP.
> >
> >Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> >
> >Index: kvm-userspace.io/qemu/hw/apic.c
> >===================================================================
> >--- kvm-userspace.io.orig/qemu/hw/apic.c
> >+++ kvm-userspace.io/qemu/hw/apic.c
> >@@ -248,8 +248,11 @@ void cpu_set_apic_base(CPUState *env, ui
> > #ifdef DEBUG_APIC
> >     printf("cpu_set_apic_base: %016" PRIx64 "\n", val);
> > #endif
> >-    s->apicbase = (val & 0xfffff000) |
> >-        (s->apicbase & (MSR_IA32_APICBASE_BSP | 
> >MSR_IA32_APICBASE_ENABLE));
> >+    if (kvm_enabled() && qemu_kvm_irqchip_in_kernel())
> >+        s->apicbase = val;
> >+    else
> >+        s->apicbase = (val & 0xfffff000) |
> >+            (s->apicbase & (MSR_IA32_APICBASE_BSP | 
> >MSR_IA32_APICBASE_ENABLE));
> >     /* if disabled, cannot be enabled again */
> >     if (!(val & MSR_IA32_APICBASE_ENABLE)) {
> >         s->apicbase &= ~MSR_IA32_APICBASE_ENABLE;
> >
> >  
> 
> Can you explain how the existing code fails?

It fails because apic_load (qemu/hw/apic.c), which runs
after cpu_load, will set the apic base value saved by
QEMU. And for some reason cpu_set_apic_base() uses the
MSR_IA32_APICBASE_BSP/MSR_IA32_APICBASE_ENABLE bits from s->apicbase
instead of what the caller passed. That breaks if in-kernel APIC
emulation is being used.

So this is the same bug which caused vmport to disable the APIC 
for SMP guests on X11.

I think it is better to revert that partial fix and instead make sure
that cpu_get_apic_base() will return the proper value.

This also fixes
https://sourceforge.net/tracker/?func=detail&atid=893831&aid=1936539&group_id=180599


QEMU/KVM: properly copy the in-kernel apicbase value

The MSR_IA32_APICBASE_ENABLE/MSR_IA32_APICBASE_BSP bits in s->apicbase
are not initialized if in-kernel APIC emulation is used, so save the 
actual value passed by cpu_set_apic_base() caller.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Index: kvm-userspace.io/qemu/hw/apic.c
===================================================================
--- kvm-userspace.io/qemu/hw/apic.c
+++ kvm-userspace.io/qemu/hw/apic.c
@@ -248,8 +248,11 @@ void cpu_set_apic_base(CPUState *env, ui
 #ifdef DEBUG_APIC
     printf("cpu_set_apic_base: %016" PRIx64 "\n", val);
 #endif
-    s->apicbase = (val & 0xfffff000) |
-        (s->apicbase & (MSR_IA32_APICBASE_BSP | MSR_IA32_APICBASE_ENABLE));
+    if (kvm_enabled() && qemu_kvm_irqchip_in_kernel())
+        s->apicbase = val;
+    else
+        s->apicbase = (val & 0xfffff000) |
+            (s->apicbase & (MSR_IA32_APICBASE_BSP | MSR_IA32_APICBASE_ENABLE));
     /* if disabled, cannot be enabled again */
     if (!(val & MSR_IA32_APICBASE_ENABLE)) {
         s->apicbase &= ~MSR_IA32_APICBASE_ENABLE;
Index: marcelo/git/kvm-userspace.io/qemu/qemu-kvm-x86.c
===================================================================
--- marcelo.orig/git/kvm-userspace.io/qemu/qemu-kvm-x86.c
+++ marcelo/git/kvm-userspace.io/qemu/qemu-kvm-x86.c
@@ -248,13 +248,8 @@ void kvm_arch_load_regs(CPUState *env)
     sregs.cr3 = env->cr[3];
     sregs.cr4 = env->cr[4];
 
-    if (kvm_irqchip_in_kernel(kvm_context)) {
-        sregs.cr8 = kvm_get_cr8(kvm_context, env->cpu_index);
-        sregs.apic_base = kvm_get_apic_base(kvm_context, env->cpu_index);
-    } else {
-        sregs.cr8 = cpu_get_apic_tpr(env);
-        sregs.apic_base = cpu_get_apic_base(env);
-    }
+    sregs.cr8 = cpu_get_apic_tpr(env);
+    sregs.apic_base = cpu_get_apic_base(env);
 
     sregs.efer = env->efer;
 

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