Re: [PATCH v2 1/7] KVM: x86/mmu: Track if shadow MMU active

[Sean Christopherson]

On Thu, Apr 29, 2021, Ben Gardon wrote:
> Add a field to each VM to track if the shadow / legacy MMU is actually
> in use. If the shadow MMU is not in use, then that knowledge opens the
> door to other optimizations which will be added in future patches.
> 
> Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  2 ++
>  arch/x86/kvm/mmu/mmu.c          | 10 +++++++++-
>  arch/x86/kvm/mmu/mmu_internal.h |  2 ++
>  arch/x86/kvm/mmu/tdp_mmu.c      |  6 ++++--
>  arch/x86/kvm/mmu/tdp_mmu.h      |  4 ++--
>  5 files changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index ad22d4839bcc..3900dcf2439e 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1122,6 +1122,8 @@ struct kvm_arch {
>  	 */
>  	spinlock_t tdp_mmu_pages_lock;
>  #endif /* CONFIG_X86_64 */
> +
> +	bool shadow_mmu_active;

I'm not a fan of the name, "shadow mmu" in KVM almost always means shadow paging
of some form, whereas this covers both shadow paging and legacy TDP support.

But, I think we we can avoid bikeshedding by simply eliminating this flag.  More
in later patches.

>  };
>  
>  struct kvm_vm_stat {
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 930ac8a7e7c9..3975272321d0 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3110,6 +3110,11 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  	return ret;
>  }
>  
> +void activate_shadow_mmu(struct kvm *kvm)
> +{
> +	kvm->arch.shadow_mmu_active = true;
> +}
> +
>  static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
>  			       struct list_head *invalid_list)
>  {
> @@ -3280,6 +3285,8 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>  		}
>  	}
>  
> +	activate_shadow_mmu(vcpu->kvm);
> +
>  	write_lock(&vcpu->kvm->mmu_lock);
>  	r = make_mmu_pages_available(vcpu);
>  	if (r < 0)
> @@ -5467,7 +5474,8 @@ void kvm_mmu_init_vm(struct kvm *kvm)
>  {
>  	struct kvm_page_track_notifier_node *node = &kvm->arch.mmu_sp_tracker;
>  
> -	kvm_mmu_init_tdp_mmu(kvm);
> +	if (!kvm_mmu_init_tdp_mmu(kvm))
> +		activate_shadow_mmu(kvm);

Doesn't come into play yet, but I would strongly prefer to open code setting the
necessary flag instead of relying on the helper to never fail.