linux-kernel - Re: [PATCH] KVM: nVMX: WARN on any attempt to allocate shadow VMCS for vmcs02

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Message-ID: <053bb241-ea71-abf8-262b-7b452dc49d37@redhat.com>
Date:   Wed, 26 Jan 2022 17:05:30 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: WARN on any attempt to allocate shadow VMCS
 for vmcs02

On 1/26/22 16:56, Vitaly Kuznetsov wrote:
>> -	WARN_ON(loaded_vmcs == &vmx->vmcs01 && loaded_vmcs->shadow_vmcs);
>> +	if (WARN_ON(loaded_vmcs != &vmx->vmcs01 || loaded_vmcs->shadow_vmcs))
>> +		return loaded_vmcs->shadow_vmcs;
> Stupid question: why do we want to care about 'loaded_vmcs' at all,
> i.e. why can't we hardcode 'vmx->vmcs01' in alloc_shadow_vmcs()? The
> only caller is enter_vmx_operation() and AFAIU 'loaded_vmcs' will always
> be pointing to 'vmx->vmcs01' (as enter_vmx_operation() allocates
> &vmx->nested.vmcs02 so 'loaded_vmcs' can't point there!).
> 

Well, that's why the WARN never happens.  The idea is that if shadow 
VMCS _virtualization_ (not emulation, i.e. running L2 VMREAD/VMWRITE 
without even a vmexit to L0) was supported, then you would need a 
non-NULL shadow_vmcs in vmx->vmcs02.

Regarding the patch, the old WARN was messy but it was also trying to 
avoid a NULL pointer dereference in the caller.

What about:

	if (WARN_ON(loaded_vmcs->shadow_vmcs))
		return loaded_vmcs->shadow_vmcs;

	/* Go ahead anyway.  */
	WARN_ON(loaded_vmcs != &vmx->vmcs01);

?

Paolo