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

Sean Christopherson
2020-03-07T00:28:55Z

On Fri, Mar 06, 2020 at 03:57:25PM -0800, Krish Sadhukhan wrote:
>
> 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 {
> >>+    VMCS,
> >>+    EVMCS,
> >>+    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 {
> +     VMXON_REGION,
> +     VMXON_VMCS,
> +     SHADOW_VMCS_REGION,
> +};
>
> ?
>
> In that case, "VMXON_REGION" and "VMXON_VMCS" are no different according to
> your explanation.

enum vmcs_type {
VMXON_VMCS,
VMCS,
SHADOW_VMCS,
};

alloc_vmcs_cpu() does more than just allocate the memory, it also
initializes the data structure, e.g. "allocate and initalize a VMXON VMCS",

> > 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);
> >>> }