Re: [PATCH v19 116/130] KVM: TDX: Silently discard SMI request

[Isaku Yamahata]

On Fri, Apr 19, 2024 at 06:52:42AM -0700,
Sean Christopherson <seanjc@xxxxxxxxxx> wrote:

> On Mon, Feb 26, 2024, isaku.yamahata@xxxxxxxxx wrote:
> > From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > 
> > TDX doesn't support system-management mode (SMM) and system-management
> > interrupt (SMI) in guest TDs.  Because guest state (vcpu state, memory
> > state) is protected, it must go through the TDX module APIs to change guest
> > state, injecting SMI and changing vcpu mode into SMM.  The TDX module
> > doesn't provide a way for VMM to inject SMI into guest TD and a way for VMM
> > to switch guest vcpu mode into SMM.
> > 
> > We have two options in KVM when handling SMM or SMI in the guest TD or the
> > device model (e.g. QEMU): 1) silently ignore the request or 2) return a
> > meaningful error.
> > 
> > For simplicity, we implemented the option 1).
> >
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > ---
> >  arch/x86/kvm/smm.h         |  7 +++++-
> >  arch/x86/kvm/vmx/main.c    | 45 ++++++++++++++++++++++++++++++++++----
> >  arch/x86/kvm/vmx/tdx.c     | 29 ++++++++++++++++++++++++
> >  arch/x86/kvm/vmx/x86_ops.h | 12 ++++++++++
> >  4 files changed, 88 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/smm.h b/arch/x86/kvm/smm.h
> > index a1cf2ac5bd78..bc77902f5c18 100644
> > --- a/arch/x86/kvm/smm.h
> > +++ b/arch/x86/kvm/smm.h
> > @@ -142,7 +142,12 @@ union kvm_smram {
> >  
> >  static inline int kvm_inject_smi(struct kvm_vcpu *vcpu)
> >  {
> > -	kvm_make_request(KVM_REQ_SMI, vcpu);
> > +	/*
> > +	 * If SMM isn't supported (e.g. TDX), silently discard SMI request.
> > +	 * Assume that SMM supported = MSR_IA32_SMBASE supported.
> > +	 */
> > +	if (static_call(kvm_x86_has_emulated_msr)(vcpu->kvm, MSR_IA32_SMBASE))
> > +		kvm_make_request(KVM_REQ_SMI, vcpu);
> >  	return 0;
> 
> No, just do what KVM already does for CONFIG_KVM_SMM=n, and return -ENOTTY.  The
> *entire* point of have a return code is to handle setups that don't support SMM.
> 
> 	if (!static_call(kvm_x86_has_emulated_msr)(vcpu->kvm, MSR_IA32_SMBASE)))
> 		return -ENOTTY;
> 
> And with that, I would drop the comment, it's pretty darn clear what "assumption"
> is being made.  In quotes because it's not an assumption, it's literally KVM's
> implementation.
> 
> And then the changelog can say "do what KVM does for CONFIG_KVM_SMM=n" without
> having to explain why we decided to do something completely arbitrary for TDX.

Ok.

> >  }
> >  
> > diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
> > index ed46e7e57c18..4f3b872cd401 100644
> > --- a/arch/x86/kvm/vmx/main.c
> > +++ b/arch/x86/kvm/vmx/main.c
> > @@ -283,6 +283,43 @@ static void vt_msr_filter_changed(struct kvm_vcpu *vcpu)
> >  	vmx_msr_filter_changed(vcpu);
> >  }
> >  
> > +#ifdef CONFIG_KVM_SMM
> > +static int vt_smi_allowed(struct kvm_vcpu *vcpu, bool for_injection)
> > +{
> > +	if (is_td_vcpu(vcpu))
> > +		return tdx_smi_allowed(vcpu, for_injection);
> 
> Adding stubs for something that TDX will never support is silly.  Bug the VM and
> return an error.
> 
> 	if (KVM_BUG_ON(is_td_vcpu(vcpu)))
> 		return -EIO;
> 
> And I wouldn't even bother with vt_* wrappers, just put that right in vmx_*().
> Same thing for everything below.

Will drop them.  Those are traces to support guest debug.  It's future topic
and we have arch.guest_state_protected check now.
-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>