linux-kernel - Re: [PATCH 50/54] KVM: x86/mmu: Optimize and clean up so called "last nonleaf level" logic

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Date:   Wed, 23 Jun 2021 20:58:19 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 50/54] KVM: x86/mmu: Optimize and clean up so called
 "last nonleaf level" logic

On Wed, Jun 23, 2021, Paolo Bonzini wrote:
> On 22/06/21 19:57, Sean Christopherson wrote:
> > +#if PTTYPE == 32
> > +	/*
> > +	 * 32-bit paging requires special handling because bit 7 is ignored if
> > +	 * CR4.PSE=0, not reserved.  Clear bit 7 in the gpte if the level is
> > +	 * greater than the last level for which bit 7 is the PAGE_SIZE bit.
> > +	 *
> > +	 * The RHS has bit 7 set iff level < (2 + PSE).  If it is clear, bit 7
> > +	 * is not reserved and does not indicate a large page at this level,
> > +	 * so clear PT_PAGE_SIZE_MASK in gpte if that is the case.
> > +	 */
> > +	gpte &= level - (PT32_ROOT_LEVEL + !!mmu->mmu_role.ext.cr4_pse);
> 
> !! is not needed and possibly slightly confusing?  (We know it's a single
> bit).

Ah, I had it backwards.  I misremembered the "!!" logic added around the
mmu_role helpers, but that was to ensure that e.g. kvm_read_cr4_bits() was
squished down into a 0/1 value when setting the mmu_role bit.