Re: [PATCH 1/5] Implement #NMI exiting for nested SVM

From: Jan Kiszka <jan.kiszka@siemens.com>
To: Alexander Graf <agraf@suse.de>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>
Subject: Re: [PATCH 1/5] Implement #NMI exiting for nested SVM
Date: Fri, 18 Sep 2009 18:01:52 +0200	[thread overview]
Message-ID: <4AB3AEF0.8020500@siemens.com> (raw)
In-Reply-To: <C29673BB-58BE-4918-90B1-4D7C838A60F1@suse.de>

Alexander Graf wrote:
> Am 18.09.2009 um 15:33 schrieb Jan Kiszka <jan.kiszka@siemens.com>:
> 
>> Alexander Graf wrote:
>>> When injecting an NMI to the l1 guest while it was running the l2  
>>> guest, we
>>> didn't #VMEXIT but just injected the NMI to the l2 guest.
>>>
>>> Let's be closer to real hardware and #VMEXIT if we're supposed to  
>>> do so.
>>>
>>> Signed-off-by: Alexander Graf <agraf@suse.de>
>>> ---
>>> arch/x86/kvm/svm.c |   38 ++++++++++++++++++++++++++++++++------
>>> 1 files changed, 32 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>>> index 9a4daca..f12a669 100644
>>> --- a/arch/x86/kvm/svm.c
>>> +++ b/arch/x86/kvm/svm.c
>>> @@ -1375,6 +1375,21 @@ static int nested_svm_check_exception(struct  
>>> vcpu_svm *svm, unsigned nr,
>>>    return nested_svm_exit_handled(svm);
>>> }
>>>
>>> +static inline int nested_svm_nmi(struct vcpu_svm *svm)
>>> +{
>>> +    if (!is_nested(svm))
>>> +        return 0;
>>> +
>>> +    svm->vmcb->control.exit_code = SVM_EXIT_NMI;
>>> +
>>> +    if (nested_svm_exit_handled(svm)) {
>>> +        nsvm_printk("VMexit -> NMI\n");
>>> +        return 1;
>>> +    }
>>> +
>>> +    return 0;
>>> +}
>>> +
>>> static inline int nested_svm_intr(struct vcpu_svm *svm)
>>> {
>>>    if (!is_nested(svm))
>>> @@ -2462,7 +2477,9 @@ static int svm_nmi_allowed(struct kvm_vcpu  
>>> *vcpu)
>>>    struct vcpu_svm *svm = to_svm(vcpu);
>>>    struct vmcb *vmcb = svm->vmcb;
>>>    return !(vmcb->control.int_state & SVM_INTERRUPT_SHADOW_MASK) &&
>>> -        !(svm->vcpu.arch.hflags & HF_NMI_MASK);
>>> +        !(svm->vcpu.arch.hflags & HF_NMI_MASK) &&
>>> +        gif_set(svm) &&
>>                ^^^^^^^^^^^^
>> I'm not claiming to be up-to-date with the SVM code around NMI
>> injection, but this addition irritates me. Can you explain why I don't
>> have to worry that a cleared IF could now defer NMI injections for L1
>> guests?
> 
> It's not about IF, but GIF, which is a special SVM addition that also  
> masks NMIs.

Ah, now I got it: That's normally a host thing but, due to nesting, we
need to consider it for L1, too. OK, something learned today. :)

> 
>>> +        !is_nested(svm);
>>> }
>>>
>>> static int svm_interrupt_allowed(struct kvm_vcpu *vcpu)
>>> @@ -2488,22 +2505,31 @@ static void enable_irq_window(struct  
>>> kvm_vcpu *vcpu)
>>>    struct vcpu_svm *svm = to_svm(vcpu);
>>>    nsvm_printk("Trying to open IRQ window\n");
>>>
>>> -    nested_svm_intr(svm);
>>> +    if (nested_svm_intr(svm))
>>> +        return;
>>>
>>>    /* In case GIF=0 we can't rely on the CPU to tell us when
>>>     * GIF becomes 1, because that's a separate STGI/VMRUN intercept.
>>>     * The next time we get that intercept, this function will be
>>>     * called again though and we'll get the vintr intercept. */
>>> -    if (gif_set(svm)) {
>>> -        svm_set_vintr(svm);
>>> -        svm_inject_irq(svm, 0x0);
>>> -    }
>>> +    if (!gif_set(svm))
>>> +        return;
>>> +
>>> +    svm_set_vintr(svm);
>>> +    svm_inject_irq(svm, 0x0);
>> The last change is pure refactoring that should not belong into this
>> patch, should it?
> 
> It went along the same function and makes the code more alike. But if  
> you feel like this really should be separate, I can split it.

I've been slapped a few times for mixing both, but I'm not the person
who will merge it.

> 
>>> }
>>>
>>> static void enable_nmi_window(struct kvm_vcpu *vcpu)
>>> {
>>>    struct vcpu_svm *svm = to_svm(vcpu);
>>>
>>> +    if (nested_svm_nmi(svm))
>>> +        return;
>>> +
>>> +    /* NMI is deferred until GIF == 1. Setting GIF will cause a  
>>> #VMEXIT */
>>> +    if (!gif_set(svm))
>>> +        return;
>>> +
>> The second half is an unrelated optimization? Then please file a
>> separate patch.
> 
> Nope, it's about not injecting NMIs while GIF is not set again.

Yes, got it.

Jan

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