linux-kernel - Re: [PATCH V2 4/5] KVM: X86: Handle implicit supervisor access with SMAP

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Date:   Tue, 15 Mar 2022 22:04:46 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Lai Jiangshan <jiangshan.ljs@...group.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH V2 4/5] KVM: X86: Handle implicit supervisor access with
 SMAP

On 3/11/22 08:03, Lai Jiangshan wrote:
> -	unsigned long not_smap = (cpl - 3) & (rflags & X86_EFLAGS_AC);
> -	int index = (pfec >> 1) +
> -		    (not_smap >> (X86_EFLAGS_AC_BIT - PFERR_RSVD_BIT + 1));
> +	bool explicit_access = !(access & PFERR_IMPLICIT_ACCESS);
> +	bool not_smap = (rflags & X86_EFLAGS_AC) && explicit_access;
> +	int index = (pfec + (!!not_smap << PFERR_RSVD_BIT)) >> 1;

Also possible:

         u64 implicit_access = access & PFERR_IMPLICIT_ACCESS;
         bool not_smap = ((rflags & X86_EFLAGS_AC) | implicit_access) == X86_EFLAGS_AC;
         int index = (pfec + (not_smap << PFERR_RSVD_BIT)) >> 1;

Paolo