linux-kernel - Re: [PATCH v3 2/2] KVM: VMX: untangle VMXON revision

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Date:   Fri, 6 Mar 2020 16:28:53 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Krish Sadhukhan <krish.sadhukhan@...cle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] KVM: VMX: untangle VMXON revision_id setting when
 using eVMCS

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);
> >>>  }