linux-kernel - Re: [PATCH v3 13/34] KVM: nSVM: Keep track of Hyper-V hv_vm_id/hv_vp

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Date:   Wed, 18 May 2022 14:25:59 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 13/34] KVM: nSVM: Keep track of Hyper-V
 hv_vm_id/hv_vp_id

Maxim Levitsky <mlevitsk@...hat.com> writes:

> On Thu, 2022-04-14 at 15:19 +0200, Vitaly Kuznetsov wrote:
>> Similar to nSVM, KVM needs to know L2's VM_ID/VP_ID and Partition
>> assist page address to handle L2 TLB flush requests.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/x86/kvm/svm/hyperv.h | 16 ++++++++++++++++
>>  arch/x86/kvm/svm/nested.c |  2 ++
>>  2 files changed, 18 insertions(+)
>> 
>> diff --git a/arch/x86/kvm/svm/hyperv.h b/arch/x86/kvm/svm/hyperv.h
>> index 7d6d97968fb9..8cf702fed7e5 100644
>> --- a/arch/x86/kvm/svm/hyperv.h
>> +++ b/arch/x86/kvm/svm/hyperv.h
>> @@ -9,6 +9,7 @@
>>  #include <asm/mshyperv.h>
>>  
>>  #include "../hyperv.h"
>> +#include "svm.h"
>>  
>>  /*
>>   * Hyper-V uses the software reserved 32 bytes in VMCB
>> @@ -32,4 +33,19 @@ struct hv_enlightenments {
>>   */
>>  #define VMCB_HV_NESTED_ENLIGHTENMENTS VMCB_SW
>>  
>> +static inline void nested_svm_hv_update_vm_vp_ids(struct kvm_vcpu *vcpu)
>> +{
>> +	struct vcpu_svm *svm = to_svm(vcpu);
>> +	struct hv_enlightenments *hve =
>> +		(struct hv_enlightenments *)svm->nested.ctl.reserved_sw;
>
> Small nitpick:
>
> Can we use this as an opportunity to rename the 'reserved_sw' to \
> 'hv_enlightenments' or something, because that is what it is?
>
> Also the reserved_sw is an array, which is confusing, since from first look,
> it looks like we have a pointer dereference here.
>

Well, that's what it is in Hyper-V world and so far we didn't give it
another meaning in KVM but in theory it is not impossible, e.g. we can
use this area to speed up nested KVM on KVM.

AMD calls this "Reserved for Host usage" so we can probably rename it to 
'reserved_host' but I'm not sure it's worth the hassle...

>
>
>> +	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
>> +
>> +	if (!hv_vcpu)
>> +		return;
>> +
>> +	hv_vcpu->nested.pa_page_gpa = hve->partition_assist_page;
>> +	hv_vcpu->nested.vm_id = hve->hv_vm_id;
>> +	hv_vcpu->nested.vp_id = hve->hv_vp_id;
>> +}
>> +
>>  #endif /* __ARCH_X86_KVM_SVM_HYPERV_H__ */
>> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
>> index bed5e1692cef..2d1a76343404 100644
>> --- a/arch/x86/kvm/svm/nested.c
>> +++ b/arch/x86/kvm/svm/nested.c
>> @@ -826,6 +826,8 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
>>  
>>  	svm->nested.nested_run_pending = 1;
>>  
>> +	nested_svm_hv_update_vm_vp_ids(vcpu);
>> +
>>  	if (enter_svm_guest_mode(vcpu, vmcb12_gpa, vmcb12, true))
>>  		goto out_exit_err;
>>  
>
> That won't work after migration, since this won't be called
> if we migrate with nested guest running.
>
>
> I think that nested_svm_hv_update_vm_vp_ids should be called 
> from enter_svm_guest_mode.
>

Oh that's a good one, thanks! This could've been a hard to debug issue.

-- 
Vitaly