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 15:45:20 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...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

On Wed, 2022-05-18 at 14:25 +0200, Vitaly Kuznetsov wrote:
> 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...

This is a very good piece of information. If AMD calls it like that,
than let it be.

It is probably not worth it to rename the field then, but I think it
is might be worth it to add this info as a comment to the KVM.

Also it might be worth it to add some wrapper function for
'struct hv_enlightenments *hve = (struct hv_enlightenments *)svm->nested.ctl.reserved_sw;'
(+ check if this area is valid - currently it is copied only when 'kvm_hv_hypercall_enabled == true').

Both would be a very low priority items to be honest.

Thanks,
Best regards,
	MaxiMm Levitsky


> 
> > 
> > > +	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.
>