Re: [PATCH v6 14/25] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs

[Maxim Levitsky]

On Wed, 2023-11-01 at 11:05 -0700, Sean Christopherson wrote:
> On Tue, Oct 31, 2023, Maxim Levitsky wrote:
> > On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 66edbed25db8..a091764bf1d2 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -133,6 +133,9 @@ static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
> > >  static void __get_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
> > >  
> > >  static DEFINE_MUTEX(vendor_module_lock);
> > > +static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu);
> > > +static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu);
> > > +
> > >  struct kvm_x86_ops kvm_x86_ops __read_mostly;
> > >  
> > >  #define KVM_X86_OP(func)					     \
> > > @@ -4372,6 +4375,22 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > >  }
> > >  EXPORT_SYMBOL_GPL(kvm_get_msr_common);
> > >  
> > > +static const u32 xstate_msrs[] = {
> > > +	MSR_IA32_U_CET, MSR_IA32_PL0_SSP, MSR_IA32_PL1_SSP,
> > > +	MSR_IA32_PL2_SSP, MSR_IA32_PL3_SSP,
> > > +};
> > > +
> > > +static bool is_xstate_msr(u32 index)
> > > +{
> > > +	int i;
> > > +
> > > +	for (i = 0; i < ARRAY_SIZE(xstate_msrs); i++) {
> > > +		if (index == xstate_msrs[i])
> > > +			return true;
> > > +	}
> > > +	return false;
> > > +}
> > 
> > The name 'xstate_msr' IMHO is not clear.
> > 
> > How about naming it 'guest_fpu_state_msrs', together with adding a comment like that:
> 
> Maybe xstate_managed_msrs?  I'd prefer not to include "guest" because the behavior
> is more a property of the architecture and/or the host kernel.  I understand where
> you're coming from, but it's the MSR *values* are part of guest state, whereas the
> check is a query on how KVM manages the MSR value, if that makes sense.
Makes sense.
> 
> And I really don't like "FPU".  I get why the the kernel uses the "FPU" terminology,
> but for this check in particular I want to tie the behavior back to the architecture,
> i.e. provide the hint that the reason why these MSRs are special is because Intel
> defined them to be context switched via XSTATE.
> 
> Actually, this is unnecesary bikeshedding to some extent, using an array is silly.
> It's easier and likely far more performant (not that that matters in this case)
> to use a switch statement.
> 
> Is this better?
> 
> /*
>  * Returns true if the MSR in question is managed via XSTATE, i.e. is context
>  * switched with the rest of guest FPU state.
>  */
> static bool is_xstate_managed_msr(u32 index)
> {
> 	switch (index) {
> 	case MSR_IA32_U_CET:
> 	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
> 		return true;
> 	default:
> 		return false;
> 	}
> }

Reasonable.

> 
> /*
>  * Read or write a bunch of msrs. All parameters are kernel addresses.
>  *
>  * @return number of msrs set successfully.
>  */
> static int __msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs *msrs,
> 		    struct kvm_msr_entry *entries,
> 		    int (*do_msr)(struct kvm_vcpu *vcpu,
> 				  unsigned index, u64 *data))
> {
> 	bool fpu_loaded = false;
> 	int i;
> 
> 	for (i = 0; i < msrs->nmsrs; ++i) {
> 		/*
> 	 	 * If userspace is accessing one or more XSTATE-managed MSRs,
> 		 * temporarily load the guest's FPU state so that the guest's
> 		 * MSR value(s) is resident in hardware, i.e. so that KVM can
> 		 * get/set the MSR via RDMSR/WRMSR.
> 	 	 */
Reasonable as well.
> 		if (vcpu && !fpu_loaded && kvm_caps.supported_xss &&
> 		    is_xstate_managed_msr(entries[i].index)) {
> 			kvm_load_guest_fpu(vcpu);
> 			fpu_loaded = true;
> 		}
> 		if (do_msr(vcpu, entries[i].index, &entries[i].data))
> 			break;
> 	}
> 	if (fpu_loaded)
> 		kvm_put_guest_fpu(vcpu);
> 
> 	return i;
> }
> 

Best regards,
	Maxim Levitsky