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

Maxim Levitsky
2022-05-18T12:47:56Z

On Wed, 2022-05-18 at 14:25 +0200, Vitaly Kuznetsov wrote:
> Maxim Levitsky <mlevitsk@redhat.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@redhat.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.
>