linux-kernel - Re: [PATCH V2 3/7] KVM: X86/MMU: Link PAE root pagetable with its children

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Date:   Thu, 26 May 2022 17:12:23 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "open list:KERNEL VIRTUAL MACHINE FOR MIPS (KVM/mips)" 
        <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        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 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH V2 3/7] KVM: X86/MMU: Link PAE root pagetable with its children

On Wed, May 18, 2022 at 12:42 AM David Matlack <dmatlack@...gle.com> wrote:

>
> Ah of course. e.g. FNAME(fetch) will call is_shadow_present_pte() on PAE
> PDPTEs.
>
> Could you also update the comment above SPTE_MMU_PRESENT_MASK? Right now it
> says: "Use bit 11, as it is ignored by all flavors of SPTEs and checking a low
> bit often generates better code than for a high bit, e.g. 56+." I think it
> would be helpful to also meniton that SPTE_MMU_PRESENT_MASK is also used in
> PDPTEs which only ignore bits 11:9.
>

Hello

Thank you for the review.

I think using BUILD_BUG_ON() in the place that requires the constraint
can avoid exploding comments in the definition since it is a build
time check and there are not too many constraints.

So I didn't change it in V3.

Or better (still using build-time check rather than comments):

#define PT_PTE_IGNORE_BITS xxxx
#define PAE_PTE_IGNORE_BITS xxxx
#define EPT_PTE_IGNORE_BITS xxxx

static_assert(PT_PTE_IGNORE_BITS & PAE_PTE_IGNORE_BITS &
              EPT_PTE_IGNORE_BITS & SPTE_MMU_PRESENT_MASK);

Thanks
Lai