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Subject: Re: [PATCH v5 7/7] KVM: SVM: hyper-v: Direct Virtual Flush support
To:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org
Cc:     "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "K. Y. Srinivasan" <kys@microsoft.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org,
        Lan Tianyu <Tianyu.Lan@microsoft.com>,
        Michael Kelley <mikelley@microsoft.com>,
        Sean Christopherson <seanjc@google.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, Wei Liu <wei.liu@kernel.org>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Maxim Levitsky <mlevitsk@redhat.com>
References: <cover.1622730232.git.viremana@linux.microsoft.com>
 <fc8d24d8eb7017266bb961e39a171b0caf298d7f.1622730232.git.viremana@linux.microsoft.com>
 <878s3c65nr.fsf@vitty.brq.redhat.com>
From:   Vineeth Pillai <viremana@linux.microsoft.com>
Message-ID: <73508c14-3ca6-1d20-8f9e-14bd966c849a@linux.microsoft.com>
Date:   Tue, 15 Jun 2021 10:24:14 -0400
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Hi Vitaly,
>> +
>> +static inline void svm_hv_update_vp_id(struct vmcb *vmcb,
>> +		struct kvm_vcpu *vcpu)
>> +{
>> +	struct hv_enlightenments *hve =
>> +		(struct hv_enlightenments *)vmcb->control.reserved_sw;
>> +
>> +	if (hve->hv_vp_id != to_hv_vcpu(vcpu)->vp_index) {
>> +		hve->hv_vp_id = to_hv_vcpu(vcpu)->vp_index;
>> +		vmcb_mark_dirty(vmcb, VMCB_HV_NESTED_ENLIGHTENMENTS);
>> +	}
> This blows up in testing when no Hyper-V context was created on a vCPU,
> e.g. when running KVM selftests (to_hv_vcpu(vcpu) is NULL when no
> Hyper-V emulation features were requested on a vCPU but
> svm_hv_update_vp_id() is called unconditionally by svm_vcpu_run()).
>
> I'll be sending a patch to fix the immediate issue but I was wondering
> why we need to call svm_hv_update_vp_id() from svm_vcpu_run() as VP
> index is unlikely to change; we can probably just call it from
> kvm_hv_set_msr() instead.
Thanks a lot for catching this.

I think you are right, updating at kvm_hv_set_msr() makes sense. I was
following the vmx logic where it also sets the vp_id in vmx_vcpu_run. But it
calls a wrapper "kvm_hv_get_vpindex" which actually checks if hv_vcpu is not
null before the assignment. I should have used that instead, my mistake.
I will look a bit more into it and send out a patch for vmx and svm 
after little
more investigation.

Thanks,
Vineeth