linux-kernel - Re: [PATCH] KVM: SVM: vNMI pending bit is V_NMI_PENDING_MASK not V_NMI_BLOCKING

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Message-ID: <cae16272-2fcb-3e9f-5ad3-50d5a7e137d1@amd.com>
Date:   Sat, 20 May 2023 11:36:36 +0530
From:   Santosh Shukla <santosh.shukla@....com>
To:     Sean Christopherson <seanjc@...gle.com>,
        "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Santosh Shukla <santosh.shukla@....com>, vkuznets@...hat.com,
        jmattson@...gle.com, thomas.lendacky@....com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: SVM: vNMI pending bit is V_NMI_PENDING_MASK not
 V_NMI_BLOCKING_MASK

Hi Sean and Maciej,

On 5/19/2023 9:21 PM, Sean Christopherson wrote:
> On Fri, May 19, 2023, Maciej S. Szmigiero wrote:
>> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
>>
>> While testing Hyper-V enabled Windows Server 2019 guests on Zen4 hardware
>> I noticed that with vCPU count large enough (> 16) they sometimes froze at
>> boot.
>> With vCPU count of 64 they never booted successfully - suggesting some kind
>> of a race condition.
>>
>> Since adding "vnmi=0" module parameter made these guests boot successfully
>> it was clear that the problem is most likely (v)NMI-related.
>>
>> Running kvm-unit-tests quickly showed failing NMI-related tests cases, like
>> "multiple nmi" and "pending nmi" from apic-split, x2apic and xapic tests
>> and the NMI parts of eventinj test.
>>
>> The issue was that once one NMI was being serviced no other NMI was allowed
>> to be set pending (NMI limit = 0), which was traced to
>> svm_is_vnmi_pending() wrongly testing for the "NMI blocked" flag rather
>> than for the "NMI pending" flag.
>>
>> Fix this by testing for the right flag in svm_is_vnmi_pending().
>> Once this is done, the NMI-related kvm-unit-tests pass successfully and
>> the Windows guest no longer freezes at boot.
>>
>> Fixes: fa4c027a7956 ("KVM: x86: Add support for SVM's Virtual NMI")
>> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
> 
> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
> 
>> ---
>>
>> It's a bit sad that no-one apparently tested the vNMI patchset with
>> kvm-unit-tests on an actual vNMI-enabled hardware...
> 
> That's one way to put it.
> 
> Santosh, what happened?  This goof was present in both v3 and v4, i.e. it wasn't
> something that we botched when applying/massaging at the last minute.  And the
> cover letters for both v3 and v4 state "Series ... tested on AMD EPYC-Genoa".

My bad that I only ran svm_test with vnmi in past using Sean's KUT branch
remotes/sean-kut/svm/vnmi_test and saw that vnmi test was passing.
Here are the logs:
---
PASS: vNMI enabled but NMI_INTERCEPT unset!
PASS: vNMI with vector 2 not injected
PASS: VNMI serviced
PASS: vnmi
--- 

However when I ran mentioned tests by Maciej, I do see the failure. Thanks for this pointing out.

Reviewed-by : Santosh Shukla <Santosh.Shukla@....com>

Best Regards,
Santosh