Re: nVMX: Shadowing of CPU_BASED_VM_EXEC_CONTROL broken

[Wanpeng Li <wanpeng.li@linux.intel.com>]

On Wed, Oct 08, 2014 at 05:07:48PM +0200, Jan Kiszka wrote:
>On 2014-10-08 12:34, Paolo Bonzini wrote:
>> Il 08/10/2014 12:29, Jan Kiszka ha scritto:
>>>>> But it would write to the vmcs02, not to the shadow VMCS; the shadow
>>>>> VMCS is active during copy_shadow_to_vmcs12/copy_vmcs12_to_shadow, and
>>>>> at no other time.  It is not clear to me how the VIRTUAL_INTR_PENDING
>>>>> bit ended up from the vmcs02 (where it is perfectly fine) to the vmcs12.
>>> Well, but somehow that bit ends up in vmcs12, that's a fact. Also that
>>> the proble disappears when shadowing is disabled. Need to think about
>>> the path again. Maybe there is just a bug, not a conceptual issue.
>> 
>> Yeah, and at this point we cannot actually exclude a processor bug.  Can
>> you check that the bit is not in the shadow VMCS just before vmrun, or
>> just after enable_irq_window?
>> 
>> Having a kvm-unit-tests testcase could also be of some help.
>
>As usual, this was a nasty race that involved some concurrent VCPUs and
>proper host load, so hard to write unit tests...
>
>diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>index 04fa1b8..d6bcaca 100644
>--- a/arch/x86/kvm/vmx.c
>+++ b/arch/x86/kvm/vmx.c
>@@ -6417,6 +6417,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
> 	const unsigned long *fields = shadow_read_write_fields;
> 	const int num_fields = max_shadow_read_write_fields;
> 
>+	preempt_disable();
>+
> 	vmcs_load(shadow_vmcs);
> 
> 	for (i = 0; i < num_fields; i++) {
>@@ -6440,6 +6442,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
> 
> 	vmcs_clear(shadow_vmcs);
> 	vmcs_load(vmx->loaded_vmcs->vmcs);
>+
>+	preempt_enable();
> }
> 
> static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
>@@ -6457,6 +6461,8 @@ static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
> 	u64 field_value = 0;
> 	struct vmcs *shadow_vmcs = vmx->nested.current_shadow_vmcs;
> 
>+	preempt_disable();
>+
> 	vmcs_load(shadow_vmcs);
> 
> 	for (q = 0; q < ARRAY_SIZE(fields); q++) {
>@@ -6483,6 +6489,8 @@ static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
> 
> 	vmcs_clear(shadow_vmcs);
> 	vmcs_load(vmx->loaded_vmcs->vmcs);
>+
>+	preempt_enable();
> }
> 
> /*
>
>No proper patch yet because there might be a smarter approach without
>using the preempt_disable() hammer. But the point is that we temporarily
>load a vmcs without updating loaded_vmcs->vmcs. Now, if some other VCPU
>is scheduling in right in the middle of this, the wrong vmcs will be
>flushed and then reloaded - e.g. a non-shadow vmcs with that interrupt
>window flag set...

If non-shadow vmcs and shadow vmcs can present in one system simultaneously? 

Regards,
Wanpeng Li 

>
>Patch is currently under heavy load testing here, but it looks very good
>as the bug was quickly reproducible before I applied it.
>
>Jan
>
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>Corporate Competence Center Embedded Linux
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