Re: [RFC PATCH v9 10/27] x86/mm: Update pte_modify, pmd_modify, and _PAGE_CHG_MASK for _PAGE_DIRTY_SW

Dave Hansen
2020-02-26T22:02:10Z

The subject really needs work. Could you think of a way to summarize
the changes here in english as opposed to just listing the symbols you
modified?

I think we could probably just auto-generate subjects for patches if the
existing one were sufficient.

On 2/5/20 10:19 AM, Yu-cheng Yu wrote:
> After the introduction of _PAGE_DIRTY_SW, pte_modify and pmd_modify need to
> set the Dirty bit accordingly: if Shadow Stack is enabled and _PAGE_RW is
> cleared, use _PAGE_DIRTY_SW; otherwise _PAGE_DIRTY_HW.

You've basically gone and written the code's if() statement in english
here. That doesn't really help me understand the patch.

> Since the Dirty bit is modify by pte_modify(), remove _PAGE_DIRTY_HW from
> PAGE_CHG_MASK.

^ modified

This is a great example of a changelog that adds very little value.
It's following the comments and doing what they say, but it's pretty
obvious that the analysis stopped there.

What *kinds* of bits are in _PAGE_CHG_MASK or not? What changed about
_PAGE_DIRTY_HW. By this definition, shouldn't _PAGE_DIRTY_SW have
technically been in this mask before this patch?

> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 62aeb118bc36..2733e7ec16b3 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -702,6 +702,14 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
> val &= _PAGE_CHG_MASK;
> val |= check_pgprot(newprot) & ~_PAGE_CHG_MASK;
> val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);
> +
> + if (pte_dirty(pte)) {
> + if (static_cpu_has(X86_FEATURE_SHSTK) && !(val & _PAGE_RW))
> + val |= _PAGE_DIRTY_SW;
> + else
> + val |= _PAGE_DIRTY_HW;
> + }
> +
> return __pte(val);
> }

OK, so this is a path we use for changing bunches of PTEs to 'newprot'.
It doesn't use the pte_*() helpers that the previous patch fixed up, so
we need a new site.

Right?

Maybe that would make good changelog text.

Also, couldn't we just have a pte_fixup() function or something that did
this logic and could be shared?

> @@ -712,6 +720,14 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
> val &= _HPAGE_CHG_MASK;
> val |= check_pgprot(newprot) & ~_HPAGE_CHG_MASK;
> val = flip_protnone_guard(oldval, val, PHYSICAL_PMD_PAGE_MASK);
> +
> + if (pmd_dirty(pmd)) {
> + if (static_cpu_has(X86_FEATURE_SHSTK) && !(val & _PAGE_RW))
> + val |= _PAGE_DIRTY_SW;
> + else
> + val |= _PAGE_DIRTY_HW;
> + }
> +
> return __pmd(val);
> }
>
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 826823df917f..e7e28bf7e919 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -150,8 +150,8 @@
> * instance, and is *not* included in this mask since
> * pte_modify() does modify it.
> */
> -#define _PAGE_CHG_MASK (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT | \
> - _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY_HW | \
> +#define _PAGE_CHG_MASK (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT | \
> + _PAGE_SPECIAL | _PAGE_ACCESSED | \
> _PAGE_SOFT_DIRTY | _PAGE_DEVMAP)
> #define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)