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Date:   Wed, 22 Jun 2022 14:19:43 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     David Matlack <dmatlack@google.com>, Marc Zyngier <maz@kernel.org>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
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        Ben Gardon <bgardon@google.com>, Peter Xu <peterx@redhat.com>,
        maciej.szmigiero@oracle.com,
        "moderated list:KERNEL VIRTUAL MACHINE FOR ARM64 (KVM/arm64)" 
        <kvmarm@lists.cs.columbia.edu>,
        "open list:KERNEL VIRTUAL MACHINE FOR MIPS (KVM/mips)" 
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        <kvm@vger.kernel.org>,
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        <kvm-riscv@lists.infradead.org>, Peter Feiner <pfeiner@google.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH v6 03/22] KVM: x86/mmu: Stop passing @direct to
 mmu_alloc_root()
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References: <20220516232138.1783324-1-dmatlack@google.com>
 <20220516232138.1783324-4-dmatlack@google.com>
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On Wed, Jun 22, 2022, Paolo Bonzini wrote:
> On 6/16/22 20:47, Sean Christopherson wrote:
> > > The argument @direct is vcpu->arch.mmu->root_role.direct, so just use
> > > that.
> > It's worth calling out that, unlike non-root page tables, it's impossible to have
> > a direct root in an indirect MMU.  I.e. provide a hint as to why there's a need to
> > pass @direct in the first place.
> > 
> 
> I suppose there's *no* need to pass direct?  Also, there's the trivial (but
> less interesting) justification that kvm_mmu_load does
> 
>         if (vcpu->arch.mmu->root_role.direct)
>                 r = mmu_alloc_direct_roots(vcpu);
>         else
>                 r = mmu_alloc_shadow_roots(vcpu);
> 
> and those are the only callers of mmu_alloc_root.

Duh, you're right, grabbing root_role.direct in mmu_alloc_root() is much better.