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

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Message-ID: <152f757d37d3fd834a06fadad18165cbf44b1b48.camel@intel.com>
Date:   Sat, 21 Aug 2021 19:20:32 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Xu, Pengfei" <pengfei.xu@...el.com>,
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        "Yu, Yu-cheng" <yu-cheng.yu@...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

+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().