Re: [PATCH 2/3] KVM: nVMX: KVM_SET_NESTED_STATE - Tear down old EVMCS state before setting new state

Aaron Lewis
2019-05-08T19:21:23Z

From: Vitaly Kuznetsov <vkuznets@redhat.com>
Date: Fri, May 3, 2019 at 3:25 AM
To: Aaron Lewis
Cc: Peter Shier, <pbonzini@redhat.com>, <rkrcmar@redhat.com>,
<jmattson@google.com>, <marcorr@google.com>, <kvm@vger.kernel.org>

> Aaron Lewis <aaronlewis@google.com> writes:
>
> > Move call to nested_enable_evmcs until after free_nested() is complete.
> >
> > Signed-off-by: Aaron Lewis <aaronlewis@google.com>
> > Reviewed-by: Marc Orr <marcorr@google.com>
> > Reviewed-by: Peter Shier <pshier@google.com>
> > ---
> > arch/x86/kvm/vmx/nested.c | 6 +++---
> > 1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > index 081dea6e211a..3b39c60951ac 100644
> > --- a/arch/x86/kvm/vmx/nested.c
> > +++ b/arch/x86/kvm/vmx/nested.c
> > @@ -5373,9 +5373,6 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
> > if (kvm_state->format != 0)
> > return -EINVAL;
> >
> > - if (kvm_state->flags & KVM_STATE_NESTED_EVMCS)
> > - nested_enable_evmcs(vcpu, NULL);
> > -
> > if (!nested_vmx_allowed(vcpu))
> > return kvm_state->vmx.vmxon_pa == -1ull ? 0 : -EINVAL;
> >
> > @@ -5417,6 +5414,9 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
> > if (kvm_state->vmx.vmxon_pa == -1ull)
> > return 0;
> >
> > + if (kvm_state->flags & KVM_STATE_NESTED_EVMCS)
> > + nested_enable_evmcs(vcpu, NULL);
> > +
> > vmx->nested.vmxon_ptr = kvm_state->vmx.vmxon_pa;
> > ret = enter_vmx_operation(vcpu);
> > if (ret)
>
> nested_enable_evmcs() doesn't do much, actually, in case it was
> previously enabled it doesn't do anything and in case it wasn't ordering
> with free_nested() (where you're aiming at nested_release_evmcs() I
> would guess) shouldn't matter. So could you please elaborate (better in
> the commit message) why do we need this re-ordered? My guess is that
> you'd like to perform checks for e.g. 'vmx.vmxon_pa == -1ull' before
> we actually start doing any changes but let's clarify that.
>
> Thanks!
>
> --
> Vitaly

There are two reasons for doing this:
1. We don't want to set new state if we are going to leave nesting and
exit the function (ie: vmx.vmxon_pa = -1), like you pointed out.
2. To be more future proof, we don't want to set new state before
tearing down state. This could cause conflicts down the road.

I can add this to the commit message if there are no objections to these points.