Re: [PATCH v3 2/2] KVM: VMX: untangle VMXON revision_id setting when using eVMCS

[Krish Sadhukhan]

On 3/6/20 3:07 PM, Sean Christopherson wrote:
> On Fri, Mar 06, 2020 at 02:20:13PM -0800, Krish Sadhukhan wrote:
> > > @@ -2599,7 +2607,7 @@ void free_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
> > >   int alloc_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
> > >   {
> > > -	loaded_vmcs->vmcs = alloc_vmcs(false);
> > > +	loaded_vmcs->vmcs = alloc_vmcs(VMCS_REGION);
> > >   	if (!loaded_vmcs->vmcs)
> > >   		return -ENOMEM;
> > > @@ -2652,25 +2660,13 @@ static __init int alloc_vmxon_regions(void)
> > >   	for_each_possible_cpu(cpu) {
> > >   		struct vmcs *vmcs;
> > > -		vmcs = alloc_vmcs_cpu(false, cpu, GFP_KERNEL);
> > > +		/* The VMXON region is really just a special type of VMCS. */
> >
> > Not sure if this is the right way to correlate the two.
> >
> > AFAIU, the SDM calls VMXON region as a memory area that holds the VMCS data
> > structure and it calls VMCS the data structure that is used by software to
> > switch between VMX root-mode and not-root-mode. So VMXON is a memory area
> > whereas VMCS is the structure of the data that resides in that memory area.
> >
> > So if we follow this interpretation, your enum should rather look like,
> >
> > enum vmcs_type {
> > +ÂÂÂ VMCS,
> > +ÂÂÂ EVMCS,
> > +ÂÂÂ SHADOW_VMCS
> No (to the EVMCS suggestion), because this allocation needs to happen for
> !eVMCS.  The SDM never explictly calls the VMXON region a VMCS, but it's
> just being coy.  E.g. VMCLEAR doesn't fail if you point it at random
> memory, but point it at the VMXON region and it yells.
>
> We could call it VMXON_VMCS if that helps.

Are you saying,

+ enum vmcs_type {
+ÂÂÂÂ VMXON_REGION,
+ÂÂÂÂ VMXON_VMCS,
+ÂÂÂÂ SHADOW_VMCS_REGION,
+};

?

In that case, "VMXON_REGION" and "VMXON_VMCS" are no different according 
to your explanation.


>   The SDM does call the memory
> allocation for regular VMCSes a "VMCS region":
>
>    A logical processor associates a region in memory with each VMCS. This
>    region is called the VMCS region.
>
> I don't think I've ever heard anyone differentiate that two though, i.e.
> VMCS is used colloquially to mean both the data structure itself and the
> memory region containing the data structure.
>
> > > +		vmcs = alloc_vmcs_cpu(VMXON_REGION, cpu, GFP_KERNEL);
> > >   		if (!vmcs) {
> > >   			free_vmxon_regions();
> > >   			return -ENOMEM;
> > >   		}
> > > -		/*
> > > -		 * When eVMCS is enabled, alloc_vmcs_cpu() sets
> > > -		 * vmcs->revision_id to KVM_EVMCS_VERSION instead of
> > > -		 * revision_id reported by MSR_IA32_VMX_BASIC.
> > > -		 *
> > > -		 * However, even though not explicitly documented by
> > > -		 * TLFS, VMXArea passed as VMXON argument should
> > > -		 * still be marked with revision_id reported by
> > > -		 * physical CPU.
> > > -		 */
> > > -		if (static_branch_unlikely(&enable_evmcs))
> > > -			vmcs->hdr.revision_id = vmcs_config.revision_id;
> > > -
> > >   		per_cpu(vmxarea, cpu) = vmcs;
> > >   	}
> > >   	return 0;
> > > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> > > index e64da06c7009..a5eb92638ac2 100644
> > > --- a/arch/x86/kvm/vmx/vmx.h
> > > +++ b/arch/x86/kvm/vmx/vmx.h
> > > @@ -489,16 +489,22 @@ static inline struct pi_desc *vcpu_to_pi_desc(struct kvm_vcpu *vcpu)
> > >   	return &(to_vmx(vcpu)->pi_desc);
> > >   }
> > > -struct vmcs *alloc_vmcs_cpu(bool shadow, int cpu, gfp_t flags);
> > > +enum vmcs_type {
> > > +	VMXON_REGION,
> > > +	VMCS_REGION,
> > > +	SHADOW_VMCS_REGION,
> > > +};
> > > +
> > > +struct vmcs *alloc_vmcs_cpu(enum vmcs_type type, int cpu, gfp_t flags);
> > >   void free_vmcs(struct vmcs *vmcs);
> > >   int alloc_loaded_vmcs(struct loaded_vmcs *loaded_vmcs);
> > >   void free_loaded_vmcs(struct loaded_vmcs *loaded_vmcs);
> > >   void loaded_vmcs_init(struct loaded_vmcs *loaded_vmcs);
> > >   void loaded_vmcs_clear(struct loaded_vmcs *loaded_vmcs);
> > > -static inline struct vmcs *alloc_vmcs(bool shadow)
> > > +static inline struct vmcs *alloc_vmcs(enum vmcs_type type)
> > >   {
> > > -	return alloc_vmcs_cpu(shadow, raw_smp_processor_id(),
> > > +	return alloc_vmcs_cpu(type, raw_smp_processor_id(),
> > >   			      GFP_KERNEL_ACCOUNT);
> > >   }