linux-kernel - Re: [PATCH 16/22] KVM: x86/mmu: remove redundant bits from extended role

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Date:   Thu, 14 Apr 2022 10:27:11 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     seanjc@...gle.com
Subject: Re: [PATCH 16/22] KVM: x86/mmu: remove redundant bits from extended
 role

On 4/14/22 09:39, Paolo Bonzini wrote:
> Before the separation of the CPU and the MMU role, CR0.PG was not
> available in the base MMU role, because two-dimensional paging always
> used direct=1 in the MMU role.  However, now that the raw role is
> snapshotted in mmu->cpu_role, CR0.PG *can* be found (though inverted)
> as !cpu_role.base.direct.  There is no need to store it again in union
> kvm_mmu_extended_role; instead, write an is_cr0_pg accessor by hand that
> takes care of the inversion.
> 
> Likewise, CR4.PAE is now always present in the CPU role as
> !cpu_role.base.has_4_byte_gpte.  The inversion makes certain tests on
> the MMU role easier, and is easily hidden by the is_cr4_pae accessor
> when operating on the CPU role.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Better:

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index cf8a41675a79..2a9b589192c3 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -234,7 +234,7 @@ BUILD_MMU_ROLE_ACCESSOR(base, efer, nx);

  static inline bool is_cr0_pg(struct kvm_mmu *mmu)
  {
-        return !mmu->cpu_role.base.direct;
+        return mmu->cpu_role.base.level > 0;
  }

  static inline bool is_cr4_pae(struct kvm_mmu *mmu)

given that the future of the direct bit is unclear.

Paolo