Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW

[Peter Zijlstra]

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).
> + */