Re: [PATCH 8/9] VMX: work around lacking VNMI support

[Jan Kiszka <jan.kiszka@web.de>]

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Gleb Natapov wrote:
> On Sun, Sep 21, 2008 at 06:57:32PM +0200, Jan Kiszka wrote:
>> Gleb Natapov wrote:
>>> Hi Jan,
>>>
>>> On Fri, Sep 19, 2008 at 02:04:37PM +0200, Jan Kiszka wrote:
>>>>  static void vmx_inject_irq(struct kvm_vcpu *vcpu, int irq)
>>>>  {
>>>>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>>>> @@ -2356,6 +2384,29 @@ static void vmx_inject_nmi(struct kvm_vc
>>>>  {
>>>>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>>>>  
>>>> +	if (!cpu_has_virtual_nmis()) {
>>>> +		int desc_size = is_long_mode(vcpu) ? 16 : 8;
>>>> +		struct descriptor_table dt;
>>>> +		gpa_t gpa;
>>>> +		u64 desc;
>>>> +
>>>> +		/*
>>>> +		 * Deny delivery if the NMI will not be handled by an
>>>> +		 * interrupt gate (workaround depends on IRQ masking).
>>>> +		 */
>>>> +		vmx_get_idt(vcpu, &dt);
>>>> +		if (!vcpu->arch.rmode.active && dt.limit
>>>> +		    >= desc_size * (NMI_VECTOR + 1) - 1) {
>>>> +			gpa = vcpu->arch.mmu.gva_to_gpa(vcpu,
>>>> +					dt.base + desc_size * NMI_VECTOR);
>>>> +			if (kvm_read_guest(vcpu->kvm, gpa, &desc, 8) == 0
>>>> +			    && ((desc >> 40) & 0x7) != 0x6)
>>>> +				return;
>>>> +		}
>>> Windows2003 sets NMI entry in IDT as a task gate (0x5) during hibernation and this check
>>> prevents it from shutting down itself. It hangs in "It is save to turn
>>> your computer now" screen.
>> Grmbl, what a weird guest...
> This is a known trick that some OSes use.

OK. Out of curiosity: What is that trick precisely?

> 
>> Is this a regression of this patch because NMIs were considered broken
>> by Windows on that host CPU so far?
>>
> Nope. This is not a regression. Hibernation hangs in the same place with
> the current git on this machine. It works on newer CPUs.
> 
> 
>>> If I replace this part by:
>>>     if(vmx->soft_vnmi_blocked)
>>> 	  return;
>>> It shut itself down properly.
>> OK, but that almost always evaluates to false here.
> Without this check guest BSODs. CPU 0 send two NMI in a row to CPU 1 and
> if second one is accepted something goes wrong.

That should have been caught at the caller site of vmx_inject_nmi
already, having to catch it here is an indication of a deeper problem.
Are you sure the NMIs are sent by CPU (i.e. kvm_inject_nmi is called
twice)? Maybe it is a bug I fixed meanwhile, an updated series goes out
later this morning.

Jan


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