Re: [Xen-devel] [PATCH for-xen-4.5] x86/pvh/vpmu: Disable VPMU for PVH guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.11.14 at 20:49, <boris.ostrovsky@xxxxxxxxxx> wrote:
> Currently when VPMU is enabled on a system both HVM and PVH VPCUs will
> initialize their VPMUs, including setting up vpmu_ops. As result even
> though VPMU will not work for PVH guests (APIC is not supported there),
> the guest may decide to perform a write to a PMU MSR. This will cause a
> call to is_vlapic_lvtpc_enabled() which will crash the hypervisor, e.g.:
> 
>  (XEN) Xen call trace:
>  (XEN)    [<ffff82d0801ca06f>] is_vlapic_lvtpc_enabled+0x13/0x22
>  (XEN)    [<ffff82d0801e2a15>] core2_vpmu_do_wrmsr+0x415/0x589
>  (XEN)    [<ffff82d0801cedaa>] vpmu_do_wrmsr+0x2a/0x33
>  (XEN)    [<ffff82d0801dd648>] vmx_msr_write_intercept+0x268/0x557
>  (XEN)    [<ffff82d0801bcd2e>] hvm_msr_write_intercept+0x36c/0x39b
>  (XEN)    [<ffff82d0801e0a0e>] vmx_vmexit_handler+0x1082/0x185b
>  (XEN)    [<ffff82d0801e74c1>] vmx_asm_vmexit_handler+0x41/0xc0
> 
> If we prevent VPMU from being initialized on PVH guests we will avoid
> those accesses.

I think this fix is too specific; instead we should mark the LAPIC
disabled, which will implicitly prevent the issue afaict - see below.

Jan

x86/PVH: properly disable vLAPIC

Rather than guarding higher level operations (like vPMU initialization
as suggested by Boris in
http://lists.xenproject.org/archives/html/xen-devel/2014-11/msg02278.html)
mark the vLAPIC hardware disabled for PVH guests and prevent it from
getting moved out of this state.

Reported-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -2217,8 +2217,7 @@ int hvm_vcpu_initialise(struct vcpu *v)
         goto fail1;
 
     /* NB: vlapic_init must be called before hvm_funcs.vcpu_initialise */
-    if ( is_hvm_vcpu(v) )
-        rc = vlapic_init(v);
+    rc = vlapic_init(v);
     if ( rc != 0 ) /* teardown: vlapic_destroy */
         goto fail2;
 
@@ -4483,7 +4482,8 @@ int hvm_msr_write_intercept(unsigned int
         break;
 
     case MSR_IA32_APICBASE:
-        if ( !vlapic_msr_set(vcpu_vlapic(v), msr_content) )
+        if ( unlikely(is_pvh_vcpu(v)) ||
+             !vlapic_msr_set(vcpu_vlapic(v), msr_content) )
             goto gp_fault;
         break;
 
--- a/xen/arch/x86/hvm/vlapic.c
+++ b/xen/arch/x86/hvm/vlapic.c
@@ -1429,6 +1429,12 @@ int vlapic_init(struct vcpu *v)
 
     HVM_DBG_LOG(DBG_LEVEL_VLAPIC, "%d", v->vcpu_id);
 
+    if ( is_pvh_vcpu(v) )
+    {
+        vlapic->hw.disabled = VLAPIC_HW_DISABLED;
+        return 0;
+    }
+
     vlapic->pt.source = PTSRC_lapic;
 
     if (vlapic->regs_page == NULL)




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