Re: [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write tracking externally

[Sean Christopherson <seanjc@google.com>]

On Wed, Apr 27, 2022, Maxim Levitsky wrote:
> This will be used to enable write tracking from nested AVIC code
> and can also be used to enable write tracking in GVT-g module
> when it actually uses it as opposed to always enabling it,
> when the module is compiled in the kernel.

Wrap at ~75.

> No functional change intended.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  arch/x86/include/asm/kvm_host.h       |  2 +-
>  arch/x86/include/asm/kvm_page_track.h |  1 +
>  arch/x86/kvm/mmu.h                    |  8 +++++---
>  arch/x86/kvm/mmu/mmu.c                | 17 ++++++++++-------
>  arch/x86/kvm/mmu/page_track.c         | 10 ++++++++--
>  5 files changed, 25 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 636df87542555..fc7df778a3d71 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1254,7 +1254,7 @@ struct kvm_arch {
>  	 * is used as one input when determining whether certain memslot
>  	 * related allocations are necessary.
>  	 */

The above comment needs to be rewritten.

> -	bool shadow_root_allocated;
> +	bool mmu_page_tracking_enabled;
>  #if IS_ENABLED(CONFIG_HYPERV)
>  	hpa_t	hv_root_tdp;
> diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
> index eb186bc57f6a9..955a5ae07b10e 100644
> --- a/arch/x86/include/asm/kvm_page_track.h
> +++ b/arch/x86/include/asm/kvm_page_track.h
> @@ -50,6 +50,7 @@ int kvm_page_track_init(struct kvm *kvm);
>  void kvm_page_track_cleanup(struct kvm *kvm);
>  
>  bool kvm_page_track_write_tracking_enabled(struct kvm *kvm);
> +int kvm_page_track_write_tracking_enable(struct kvm *kvm);
>  int kvm_page_track_write_tracking_alloc(struct kvm_memory_slot *slot);
>  
>  void kvm_page_track_free_memslot(struct kvm_memory_slot *slot);
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 671cfeccf04e9..44d15551f7156 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -269,7 +269,7 @@ int kvm_arch_write_log_dirty(struct kvm_vcpu *vcpu);
>  int kvm_mmu_post_init_vm(struct kvm *kvm);
>  void kvm_mmu_pre_destroy_vm(struct kvm *kvm);
>  
> -static inline bool kvm_shadow_root_allocated(struct kvm *kvm)
> +static inline bool mmu_page_tracking_enabled(struct kvm *kvm)
>  {
>  	/*
>  	 * Read shadow_root_allocated before related pointers. Hence, threads
> @@ -277,9 +277,11 @@ static inline bool kvm_shadow_root_allocated(struct kvm *kvm)
>  	 * see the pointers. Pairs with smp_store_release in
>  	 * mmu_first_shadow_root_alloc.
>  	 */

This comment also needs to be rewritten.

> -	return smp_load_acquire(&kvm->arch.shadow_root_allocated);
> +	return smp_load_acquire(&kvm->arch.mmu_page_tracking_enabled);
>  }

...

> diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
> index 2e09d1b6249f3..8857d629036d7 100644
> --- a/arch/x86/kvm/mmu/page_track.c
> +++ b/arch/x86/kvm/mmu/page_track.c
> @@ -21,10 +21,16 @@
>  
>  bool kvm_page_track_write_tracking_enabled(struct kvm *kvm)

This can be static, it's now used only by page_track.c.

>  {
> -	return IS_ENABLED(CONFIG_KVM_EXTERNAL_WRITE_TRACKING) ||
> -	       !tdp_enabled || kvm_shadow_root_allocated(kvm);
> +	return mmu_page_tracking_enabled(kvm);
>  }
>  
> +int kvm_page_track_write_tracking_enable(struct kvm *kvm)

This is too similar to the "enabled" version; "kvm_page_track_enable_write_tracking()"
would maintain namespacing and be less confusing.

Hmm, I'd probably vote to make this a "static inline" in kvm_page_track.h, and
rename mmu_enable_write_tracking() to kvm_mmu_enable_write_tracking and export.
Not a strong preference, just feels silly to export a one-liner.

> +{
> +	return mmu_enable_write_tracking(kvm);
> +}
> +EXPORT_SYMBOL_GPL(kvm_page_track_write_tracking_enable);
> +
> +
>  void kvm_page_track_free_memslot(struct kvm_memory_slot *slot)
>  {
>  	int i;
> -- 
> 2.26.3
>