linux-kernel - Re: [PATCH v29 09/32] x86/mm: Introduce _PAGE

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Date:   Sun, 22 Aug 2021 02:59:54 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Xu, Pengfei" <pengfei.xu@...el.com>,
        "Huang, Haitao" <haitao.huang@...el.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "esyr@...hat.com" <esyr@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "bsingharora@...il.com" <bsingharora@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "nadav.amit@...il.com" <nadav.amit@...il.com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
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        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
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        "Dave.Martin@....com" <Dave.Martin@....com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
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CC:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v29 09/32] x86/mm: Introduce _PAGE_COW

On Sat, 2021-08-21 at 19:20 +0000, Edgecombe, Rick P wrote:
> +KVM list.
> 
> On Fri, 2021-08-20 at 11:11 -0700, Yu-cheng Yu wrote:
> >  
> >  static inline int pte_write(pte_t pte)
> >  {
> > -       return pte_flags(pte) & _PAGE_RW;
> > +       /*
> > +        * Shadow stack pages are always writable - but not by
> > normal
> > +        * instructions, and only by shadow stack operations. 
> > Therefore,
> > +        * the W=0,D=1 test with pte_shstk().
> > +        */
> > +       return (pte_flags(pte) & _PAGE_RW) || pte_shstk(pte);
> >  }
> >  
> 
> KVM uses this in a couple places when checking EPT ptes. But bit 6
> (dirty) is a totally different meaning in EPT. I think it's just used
> to trigger an optimization, but wondering if KVM should have its own
> TDP specific function instead of using pte_write().
> 
Argh, never mind. I misread the new KVM mmu notifier refactor.