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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Aaron Lewis <aaronlewis@google.com> writes:

> 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.

Sounds good to me, please do. Thanks!

-- 
Vitaly