linux-kernel - Re: [PATCH v2 08/15] KVM: x86/tdp_mmu: Introduce KVM MMU root types to specify page table type

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Date: Fri, 7 Jun 2024 20:06:42 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>, "sagis@...gle.com" <sagis@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Aktas, Erdem"
	<erdemaktas@...gle.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"dmatlack@...gle.com" <dmatlack@...gle.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>
Subject: Re: [PATCH v2 08/15] KVM: x86/tdp_mmu: Introduce KVM MMU root types
 to specify page table type

On Fri, 2024-06-07 at 10:10 +0200, Paolo Bonzini wrote:
> On Thu, May 30, 2024 at 11:07 PM Rick Edgecombe
> <rick.p.edgecombe@...el.com> wrote:
> > +enum kvm_tdp_mmu_root_types {
> > +       KVM_VALID_ROOTS = BIT(0),
> > +
> > +       KVM_ANY_ROOTS = 0,
> > +       KVM_ANY_VALID_ROOTS = KVM_VALID_ROOTS,
> 
> I would instead define it as
> 
>     KVM_INVALID_ROOTS = BIT(0),
>     KVM_VALID_ROOTS = BIT(1),
>     KVM_ALL_ROOTS = KVM_VALID_ROOTS | KVM_INVALID_ROOTS,
> 
> and then
> 
>   if (root->role.invalid)
>     return types & KVM_INVALID_ROOTS;
>   else
>     return types & KVM_VALID_ROOTS;
> 
> Then in the next patch you can do
> 
>      KVM_INVALID_ROOTS = BIT(0),
> -    KVM_VALID_ROOTS = BIT(1),
> +   KVM_DIRECT_ROOTS = BIT(1),
> +   KVM_MIRROR_ROOTS = BIT(2),
> +   KVM_VALID_ROOTS = KVM_DIRECT_ROOTS | KVM_MIRROR_ROOTS,
>      KVM_ALL_ROOTS = KVM_VALID_ROOTS | KVM_INVALID_ROOTS,
> 
> and likewise
> 
>   if (root->role.invalid)
>     return types & KVM_INVALID_ROOTS;
>   else if (likely(!is_mirror_sp(root)))
>     return types & KVM_DIRECT_ROOTS;
>   else
>     return types & KVM_MIRROR_ROOTS;
> 
> This removes the need for KVM_ANY_VALID_ROOTS (btw I don't know if
> it's me, but ALL sounds more grammatical than ANY in this context). So
> the resulting code should be a bit clearer.
> 
> Apart from this small tweak, the overall idea is really good.

Yes, this makes more sense. Thanks.