Re: [PATCH 2/2] KVM: VMX: Inject #GP, not #UD, if SGX2 ENCLS leafs are unsupported

[Sean Christopherson]

On Thu, Apr 06, 2023, Huang, Kai wrote:
> On Wed, 2023-04-05 at 16:45 -0700, Sean Christopherson wrote:
> > Per Intel's SDM, unsupported ENCLS leafs result in a #GP, not a #UD.
> > SGX1 is a special snowflake as the SGX1 flag is used by the CPU as a
> > "soft" disable, e.g. if software disables machine check reporting, i.e.
> > having SGX but not SGX1 is effectively "SGX completely unsupported" and
> > and thus #UDs.
> 
> If I recall correctly, this is an erratum which can clear SGX1 in CPUID while
> the SGX flag is still in CPUID?

Nope, not an erratum, architectural behavior.

> But I am not sure whether this part is relevant to this patch?  Because SDM
> already says ENCLS causes #UD if SGX1 isn't present.  This patch changes
> "unsupported leaf" from causing #UD to causing #GP, which is also documented in
> SDM.

I wanted to capture why SGX1 is different and given special treatment in the SDM.
I.e. to explain why SGX1 leafs are an exception to the "#GP if leaf unsupported"
clause.

> > Fixes: 9798adbc04cf ("KVM: VMX: Frame in ENCLS handler for SGX virtualization")
> > Cc: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
> > Cc: Kai Huang <kai.huang@xxxxxxxxx>
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/vmx/sgx.c | 15 +++++++++------
> >  1 file changed, 9 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/sgx.c b/arch/x86/kvm/vmx/sgx.c
> > index f881f6ff6408..1c092ab89c33 100644
> > --- a/arch/x86/kvm/vmx/sgx.c
> > +++ b/arch/x86/kvm/vmx/sgx.c
> > @@ -350,11 +350,12 @@ static int handle_encls_einit(struct kvm_vcpu *vcpu)
> >  
> >  static inline bool encls_leaf_enabled_in_guest(struct kvm_vcpu *vcpu, u32 leaf)
> >  {
> > -	if (!enable_sgx || !guest_cpuid_has(vcpu, X86_FEATURE_SGX))
> > -		return false;
> > -
> > +	/*
> > +	 * ENCLS #UDs if SGX1 isn't supported, i.e. this point will be reached
> 
> Why #UDs instead of #UD?  Is #UD a verb?

Heh, it is now ;-)  I can reword to something like

	/*
	 * ENCLS generates a #UD if SGX1 isn't supported ...
	 */

if my made up grammar is confusing.

> > +	 * if and only if the SGX1 leafs are enabled.
> > +	 */
> 
> Is it better to move "ENCLS #UDs if SGX1 isn't supported" part to ...
> 
> >  	if (leaf >= ECREATE && leaf <= ETRACK)
> > -		return guest_cpuid_has(vcpu, X86_FEATURE_SGX1);
> > +		return true;
> >  
> >  	if (leaf >= EAUG && leaf <= EMODT)
> >  		return guest_cpuid_has(vcpu, X86_FEATURE_SGX2);
> > @@ -373,9 +374,11 @@ int handle_encls(struct kvm_vcpu *vcpu)
> >  {
> >  	u32 leaf = (u32)kvm_rax_read(vcpu);
> >  
> > -	if (!encls_leaf_enabled_in_guest(vcpu, leaf)) {
> > +	if (!enable_sgx || !guest_cpuid_has(vcpu, X86_FEATURE_SGX) ||
> > +	    !guest_cpuid_has(vcpu, X86_FEATURE_SGX1)) {
> >  		kvm_queue_exception(vcpu, UD_VECTOR);
> 
> ... above here, where the actual code reside?

My goal was to document why encls_leaf_enabled_in_guest() unconditionally returns
true for SGX1 leafs, i.e. why it doesn't query X86_FEATURE_SGX1.  I'm definitely
not opposed to also adding a comment here, but I do want to keep the comment in
encls_leaf_enabled_in_guest().