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Subject: Re: [PATCH 0/3] KVM: x86/mmu: Add macro for hugepage GFN mask
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Ben Gardon <bgardon@google.com>
References: <20201027214300.1342-1-sean.j.christopherson@intel.com>
 <80038ae1-d603-dc22-1997-1ad7da0a936d@redhat.com>
 <20201028152948.GA7584@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <e3d68b2b-2af6-04ce-c5f6-47786d9a15bb@redhat.com>
Date:   Thu, 29 Oct 2020 08:08:48 +0100
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On 28/10/20 16:29, Sean Christopherson wrote:
> The naming and usage also aligns with the kernel, which defines PAGE, PMD and
> PUD masks, and has near identical usage patterns.
> 
>   #define PAGE_SIZE               (_AC(1,UL) << PAGE_SHIFT)
>   #define PAGE_MASK               (~(PAGE_SIZE-1))
> 
>   #define PMD_PAGE_SIZE           (_AC(1, UL) << PMD_SHIFT)
>   #define PMD_PAGE_MASK           (~(PMD_PAGE_SIZE-1))
> 
>   #define PUD_PAGE_SIZE           (_AC(1, UL) << PUD_SHIFT)
>   #define PUD_PAGE_MASK           (~(PUD_PAGE_SIZE-1))

Well, PAGE_MASK is also one of my pet peeves for Linux.  At least I am
consistent. :)

>> and of course if you're debugging it you have to
>> look closer and check if it's really "x & -y" or "x & ~y", but at least
>> in normal cursory code reading that's how it works for me.
> 
> IMO, "x & -y" has a higher barrier to entry, especially when the kernel's page
> masks uses "x & ~(y - 1))".  But, my opinion is definitely colored by my
> inability to read two's-complement on the fly.

Fair enough.  What about having instead

#define KVM_HPAGE_GFN_BASE(gfn, level)  \
   (x & ~(KVM_PAGES_PER_HPAGE(gfn) - 1))
#define KVM_HPAGE_GFN_INDEX(gfn, level)  \
   (x & (KVM_PAGES_PER_HPAGE(gfn) - 1))

?

Paolo