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

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

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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...

Is this a regression of this patch because NMIs were considered broken
by Windows on that host CPU so far?

> 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.

I guess (hope) Windows will switch to an NMI task which has IRQs
disabled (!(TSS.EFLAGS & IF)), so this may become another check for
those weird task users. Waiting on the next IRQ window for NMI injection
should work equally well with tasks, given they disable IRQs properly.

Thanks for testing!

Jan

PS: There are more issues in my v1 series, update will follow tomorrow.


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