From: "Shukla, Santosh" <santosh.shukla@amd.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
Maxim Levitsky <mlevitsk@redhat.com>,
Tom Lendacky <thomas.lendacky@amd.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv2 4/7] KVM: SVM: Report NMI not allowed when Guest busy handling VNMI
Date: Fri, 29 Jul 2022 11:21:58 +0530 [thread overview]
Message-ID: <20c2142a-ec88-02cf-01f2-cf7f8dfcef77@amd.com> (raw)
In-Reply-To: <Yth5hl+RlTaa5ybj@google.com>
Hello Sean,
On 7/21/2022 3:24 AM, Sean Christopherson wrote:
> On Sat, Jul 09, 2022, Santosh Shukla wrote:
>> In the VNMI case, Report NMI is not allowed when the processor set the
>> V_NMI_MASK to 1 which means the Guest is busy handling VNMI.
>>
>> Signed-off-by: Santosh Shukla <santosh.shukla@amd.com>
>> ---
>> v2:
>> - Moved vnmi check after is_guest_mode() in func _nmi_blocked().
>> - Removed is_vnmi_mask_set check from _enable_nmi_window().
>> as it was a redundent check.
>>
>> arch/x86/kvm/svm/svm.c | 6 ++++++
>> 1 file changed, 6 insertions(+)
>>
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index 3574e804d757..44c1f2317b45 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -3480,6 +3480,9 @@ bool svm_nmi_blocked(struct kvm_vcpu *vcpu)
>> if (is_guest_mode(vcpu) && nested_exit_on_nmi(svm))
>> return false;
>>
>> + if (is_vnmi_enabled(svm) && is_vnmi_mask_set(svm))
>> + return true;
>> +
>> ret = (vmcb->control.int_state & SVM_INTERRUPT_SHADOW_MASK) ||
>> (vcpu->arch.hflags & HF_NMI_MASK);
>>
>> @@ -3609,6 +3612,9 @@ static void svm_enable_nmi_window(struct kvm_vcpu *vcpu)
>> {
>> struct vcpu_svm *svm = to_svm(vcpu);
>>
>> + if (is_vnmi_enabled(svm))
>> + return;
>
> Ugh, is there really no way to trigger an exit when NMIs become unmasked? Because
> if there isn't, this is broken for KVM.
>
Yes. there is.
NMI_INTERCEPT will trigger VMEXIT when second NMI arrives while guest is busy handling first NMI.
And in that scenario, Guest will exit with V_NMI_MASK set to 1, KVM can inject pending(Second)
NMI(V_NMI=1). Guest will resume handling the first NMI, then HW will
clear the V_NMI_MASK and later HW will take the pending V_NMI in side the guest.
I'll handle above case in v3.
Thanks,
Santosh
> On bare metal, if two NMIs arrive "simultaneously", so long as NMIs aren't blocked,
> the first NMI will be delivered and the second will be pended, i.e. software will
> see both NMIs. And if that doesn't hold true, the window for a true collision is
> really, really tiny.
>
> But in KVM, because a vCPU may not be run a long duration, that window becomes
> very large. To not drop NMIs and more faithfully emulate hardware, KVM allows two
> NMIs to be _pending_. And when that happens, KVM needs to trigger an exit when
> NMIs become unmasked _after_ the first NMI is injected.
>
>> +
>> if ((vcpu->arch.hflags & (HF_NMI_MASK | HF_IRET_MASK)) == HF_NMI_MASK)
>> return; /* IRET will cause a vm exit */
>>
>> --
>> 2.25.1
>>
next prev parent reply other threads:[~2022-07-29 5:52 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-09 13:42 [PATCHv2 0/7] Virtual NMI feature Santosh Shukla
2022-07-09 13:42 ` [PATCHv2 1/7] x86/cpu: Add CPUID feature bit for VNMI Santosh Shukla
2022-07-09 13:42 ` [PATCHv2 2/7] KVM: SVM: Add VNMI bit definition Santosh Shukla
2022-07-09 13:42 ` [PATCHv2 3/7] KVM: SVM: Add VNMI support in get/set_nmi_mask Santosh Shukla
2022-07-10 16:15 ` Maxim Levitsky
2022-07-21 9:34 ` Shukla, Santosh
2022-07-21 12:01 ` Maxim Levitsky
2022-07-21 13:12 ` Shukla, Santosh
2022-07-21 15:48 ` Maxim Levitsky
2022-07-09 13:42 ` [PATCHv2 4/7] KVM: SVM: Report NMI not allowed when Guest busy handling VNMI Santosh Shukla
2022-07-20 21:54 ` Sean Christopherson
2022-07-21 12:05 ` Maxim Levitsky
2022-07-21 14:59 ` Sean Christopherson
2022-07-21 15:31 ` Maxim Levitsky
2022-07-21 16:08 ` Sean Christopherson
2022-07-21 16:17 ` Maxim Levitsky
2022-07-21 16:25 ` Sean Christopherson
2022-07-29 5:51 ` Shukla, Santosh [this message]
2022-07-29 14:41 ` Sean Christopherson
2022-08-04 9:51 ` Shukla, Santosh
2022-07-09 13:42 ` [PATCHv2 5/7] KVM: SVM: Add VNMI support in inject_nmi Santosh Shukla
2022-07-20 21:41 ` Sean Christopherson
2022-07-20 22:46 ` Jim Mattson
2022-07-20 23:04 ` Sean Christopherson
2022-07-29 6:06 ` Shukla, Santosh
2022-07-29 13:53 ` Sean Christopherson
2022-07-29 13:55 ` Shukla, Santosh
2022-07-09 13:42 ` [PATCHv2 6/7] KVM: nSVM: implement nested VNMI Santosh Shukla
2022-07-09 13:42 ` [PATCHv2 7/7] KVM: SVM: Enable VNMI feature Santosh Shukla
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