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

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Date:   Fri, 6 Mar 2020 15:57:25 -0800
From:   Krish Sadhukhan <krish.sadhukhan@...cle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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 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.


>   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);
>>>   }