linux-kernel - Re: [PATCH v6 09/18] KVM: x86/tdp_mmu: Extract root invalid check from tdx_mmu_next

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Message-ID: <Z4r-Znz1GQ2E1vMX@google.com>
Date: Fri, 17 Jan 2025 17:05:42 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, yan.y.zhao@...el.com, 
	isaku.yamahata@...el.com, binbin.wu@...ux.intel.com, 
	rick.p.edgecombe@...el.com
Subject: Re: [PATCH v6 09/18] KVM: x86/tdp_mmu: Extract root invalid check
 from tdx_mmu_next_root()

On Sun, Dec 22, 2024, Paolo Bonzini wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> Extract tdp_mmu_root_match() to check if the root has given types and use
> it for the root page table iterator.  It checks only_invalid now.
> 
> TDX KVM operates on a shared page table only (Shared-EPT), a mirrored page
> table only (Secure-EPT), or both based on the operation.  KVM MMU notifier
> operations only on shared page table.  KVM guest_memfd invalidation
> operations only on mirrored page table, and so on.  Introduce a centralized
> matching function instead of open coding matching logic in the iterator.
> The next step is to extend the function to check whether the page is shared
> or private
> 
> Link: https://lore.kernel.org/kvm/ZivazWQw1oCU8VBC@google.com/
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Message-ID: <20240718211230.1492011-10-rick.p.edgecombe@...el.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 5b01038ddce8..e0ccfdd4200b 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -92,6 +92,14 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
>  	call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
>  }
>  
> +static bool tdp_mmu_root_match(struct kvm_mmu_page *root, bool only_valid)
> +{
> +	if (only_valid && root->role.invalid)
> +		return false;
> +
> +	return true;

Ugh, this is almost as bad as

	if (x)
		return true;

	return false;

Just do

	return !only_valid || root->role.invalid;

And I vote to drop the helper, "match" makes me think about things like roles
and addresses, not just if a root is valid.

> +}
> +
>  /*
>   * Returns the next root after @prev_root (or the first root if @prev_root is
>   * NULL).  A reference to the returned root is acquired, and the reference to
> @@ -125,7 +133,7 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
>  						   typeof(*next_root), link);
>  
>  	while (next_root) {
> -		if ((!only_valid || !next_root->role.invalid) &&
> +		if (tdp_mmu_root_match(next_root, only_valid) &&

E.g. this can just as easily be 

		    !only_valid || root->role.invalid &&

which is amusingly even fewer characters :-D

>  		    kvm_tdp_mmu_get_root(next_root))
>  			break;
>  
> @@ -176,7 +184,7 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
>  	list_for_each_entry(_root, &_kvm->arch.tdp_mmu_roots, link)		\
>  		if (kvm_lockdep_assert_mmu_lock_held(_kvm, false) &&		\
>  		    ((_as_id >= 0 && kvm_mmu_page_as_id(_root) != _as_id) ||	\
> -		     ((_only_valid) && (_root)->role.invalid))) {		\
> +		     !tdp_mmu_root_match((_root), (_only_valid)))) {		\
>  		} else
>  
>  #define for_each_tdp_mmu_root(_kvm, _root, _as_id)			\
> -- 
> 2.43.5
> 
>