linux-kernel - Re: [PATCH v3 17/17] KVM: x86/tdp_mmu: Take root types for kvm_tdp_mmu_invalidate_all

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Date: Wed, 26 Jun 2024 10:25:47 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "Gao, Chao" <chao.gao@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"Huang, Kai" <kai.huang@...el.com>, "sagis@...gle.com" <sagis@...gle.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Aktas, Erdem"
	<erdemaktas@...gle.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v3 17/17] KVM: x86/tdp_mmu: Take root types for
 kvm_tdp_mmu_invalidate_all_roots()

On Wed, Jun 26, 2024 at 04:56:33AM +0800, Edgecombe, Rick P wrote:
> On Tue, 2024-06-25 at 14:14 +0800, Yan Zhao wrote:
> > > Explain why not to zap invalid mirrored roots?
> > No. Explain why not to zap invalid direct roots.
> > Just passing KVM_DIRECT_ROOTS will zap only valid direct roots, right?
> > The original kvm_tdp_mmu_zap_all() "Zap all roots, including invalid roots".
> > It might be better to explain why not to zap invalid direct roots here.
> 
> Hmm, right. We don't actually have enum value to iterate all direct roots (valid
> and invalid). We could change tdp_mmu_root_match() to:
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 8320b093fef9..bcd771a8835f 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -98,8 +98,8 @@ static bool tdp_mmu_root_match(struct kvm_mmu_page *root,
>         if (WARN_ON_ONCE(!(types & KVM_VALID_ROOTS)))
>                 return false;
>  
> -       if (root->role.invalid)
> -               return types & KVM_INVALID_ROOTS;
> +       if (root->role.invalid && !(types & KVM_INVALID_ROOTS))
> +               return false;
>         if (likely(!is_mirror_sp(root)))
>                 return types & KVM_DIRECT_ROOTS;
>  
> Maybe better than a comment...? Need to put the whole thing together and test it
> still.
Hmm, I think passing KVM_DIRECT_ROOTS only in kvm_tdp_mmu_zap_all() is ok,
because kvm_mmu_uninit_tdp_mmu() will zap all invalidated roots eventually,
though KVM_DIRECT_ROOTS | KVM_INVALID_ROOTS matches more to the function name
zap_all.

I'm not convinced that the change in tdp_mmu_root_match() above is needed.
Once a root is marked invalid, it won't be restored later. Distinguishing
between invalid direct roots and invalid mirror roots will only result in more
unused roots lingering unnecessarily.