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Date:   Fri, 14 Oct 2022 11:42:22 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
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        Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW
Message-ID: <Y0ku/kgLrs77rGxz@hirez.programming.kicks-ass.net>
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-11-rick.p.edgecombe@intel.com>
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On Thu, Sep 29, 2022 at 03:29:07PM -0700, Rick Edgecombe wrote:
> @@ -300,6 +324,44 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
>  	return native_make_pte(v & ~clear);
>  }
>  
> +/*
> + * Normally the Dirty bit is used to denote COW memory on x86. But

This is misleading; this isn't an x86 specific thing. The core-mm code
does this.

> + * in the case of X86_FEATURE_SHSTK, the software COW bit is used,
> + * since the Dirty=1,Write=0 will result in the memory being treated
> + * as shaodw stack by the HW. So when creating COW memory, a software
> + * bit is used _PAGE_BIT_COW. The following functions pte_mkcow() and
> + * pte_clear_cow() take a PTE marked conventially COW (Dirty=1) and
> + * transition it to the shadow stack compatible version of COW (Cow=1).
> + */