linux-kernel - Re: [PATCH v4 2/6] KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled

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Message-ID: <5aec4689-fe63-abff-94d4-8e42cf5bba66@gmail.com>
Date:   Tue, 9 May 2023 09:04:38 +0800
From:   Robert Hoo <robert.hoo.linux@...il.com>
To:     Mathias Krause <minipli@...ecurity.net>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v4 2/6] KVM: x86: Do not unload MMU roots when only
 toggling CR0.WP with TDP enabled

On 5/8/2023 5:30 PM, Mathias Krause wrote:
>>>    void kvm_post_set_cr0(struct kvm_vcpu *vcpu, unsigned long old_cr0,
>>> unsigned long cr0)
>>>    {
>>> +    /*
>>> +     * CR0.WP is incorporated into the MMU role, but only for
>>> non-nested,
>>> +     * indirect shadow MMUs.  If TDP is enabled, the MMU's metadata
>>> needs
>>> +     * to be updated, e.g. so that emulating guest translations does the
>>> +     * right thing, but there's no need to unload the root as CR0.WP
>>> +     * doesn't affect SPTEs.
>>> +     */
>>> +    if (tdp_enabled && (cr0 ^ old_cr0) == X86_CR0_WP) {
>>
>> Curiously, this patch only affects tdp_enabled, why does legacy MMU also
>> see comparable performance gains?
> 
> Because 'tdp_enabled' just implies EPT / NPT and only 'tdp_mmu_enabled'
> decides which MMU mode to use -- either legacy or TDP MMU (see
> kvm_configure_mmu() and now gets invoked from vmx.c / svm.c).
> 
Ah, get it, thanks. The name indeed confuses me (and perhaps others).
After dig into,
1. kvm modules has a param "tdp_mmu_enabled", (in the first place) 
indicates KVM level's willingness on enable two dimensional paging. 
However, it in the end depends on ept/npt enabled or not on vendor layer.
So, uses a "tdp_mmu_allowed" to intermediately record this willness in kvm 
module init phase.
	/*
	 * Snapshot userspace's desire to enable the TDP MMU. Whether or not the
	 * TDP MMU is actually enabled is determined in kvm_configure_mmu()
	 * when the vendor module is loaded.
	 */
	tdp_mmu_allowed = tdp_mmu_enabled;
2. When vendor module init --> kvm_configure_mmu()
	tdp_mmu_enabled = tdp_mmu_allowed && tdp_enabled;

    tdp_mmu_enabled's semantics becomes, as its name indicates, the 
eventual tdp mmu enablement status.

    And, tdp_enabled, is the general (ept_enabled | npt_enabled).