linux-kernel - Re: [PATCH v3 04/21] arm64: decouple early fixmap init from linear mapping

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Message-ID: <CAKv+Gu-SkF3Tf496meuhuy63AKvp7Sq=0BdnwsL1Eh+K=3Yd5w@mail.gmail.com>
Date:	Mon, 11 Jan 2016 18:15:56 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Kees Cook <keescook@...omium.org>, Arnd Bergmann <arnd@...db.de>,
	kernel-hardening@...ts.openwall.com,
	Sharma Bhupesh <bhupesh.sharma@...escale.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Stuart Yoder <stuart.yoder@...escale.com>,
	Marc Zyngier <marc.zyngier@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 04/21] arm64: decouple early fixmap init from linear mapping

On 11 January 2016 at 18:08, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> On 11 January 2016 at 17:51, Mark Rutland <mark.rutland@....com> wrote:
>> On Mon, Jan 11, 2016 at 04:27:38PM +0000, Mark Rutland wrote:
>>> On Mon, Jan 11, 2016 at 05:15:13PM +0100, Ard Biesheuvel wrote:
>>> > On 11 January 2016 at 17:09, Mark Rutland <mark.rutland@....com> wrote:
>>> > > On Mon, Jan 11, 2016 at 02:18:57PM +0100, Ard Biesheuvel wrote:
>>> > >> Since the early fixmap page tables are populated using pages that are
>>> > >> part of the static footprint of the kernel, they are covered by the
>>> > >> initial kernel mapping, and we can refer to them without using __va/__pa
>>> > >> translations, which are tied to the linear mapping.
>>> > >>
>>> > >> Since the fixmap page tables are disjoint from the kernel mapping up
>>> > >> to the top level pgd entry, we can refer to bm_pte[] directly, and there
>>> > >> is no need to walk the page tables and perform __pa()/__va() translations
>>> > >> at each step.
>>> > >>
>>> > >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>>> > >> ---
>>> > >>  arch/arm64/mm/mmu.c | 32 ++++++--------------
>>> > >>  1 file changed, 9 insertions(+), 23 deletions(-)
>>> > >>
>>> > >> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>> > >> index 7711554a94f4..75b5f0dc3bdc 100644
>>> > >> --- a/arch/arm64/mm/mmu.c
>>> > >> +++ b/arch/arm64/mm/mmu.c
>>> > >> @@ -570,38 +570,24 @@ void vmemmap_free(unsigned long start, unsigned long end)
>>> > >>  #endif       /* CONFIG_SPARSEMEM_VMEMMAP */
>>> > >>
>>> > >>  static pte_t bm_pte[PTRS_PER_PTE] __page_aligned_bss;
>>> > >> -#if CONFIG_PGTABLE_LEVELS > 2
>>> > >>  static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss;
>>> > >> -#endif
>>> > >> -#if CONFIG_PGTABLE_LEVELS > 3
>>> > >>  static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss;
>>> > >> -#endif
>>> > >>
>>> > >>  static inline pud_t * fixmap_pud(unsigned long addr)
>>> > >>  {
>>> > >> -     pgd_t *pgd = pgd_offset_k(addr);
>>> > >> -
>>> > >> -     BUG_ON(pgd_none(*pgd) || pgd_bad(*pgd));
>>> > >> -
>>> > >> -     return pud_offset(pgd, addr);
>>> > >> +     return (CONFIG_PGTABLE_LEVELS > 3) ? &bm_pud[pud_index(addr)]
>>> > >> +                                        : (pud_t *)pgd_offset_k(addr);
>>> > >
>>> > > If we move patch 6 earlier, we could use pud_offset_kimg here, and avoid
>>> > > the cast, at the cost of passing the pgd into fixmap_pud.
>>> > >
>>> > > Similarly for fixmap_pmd.
>>> > >
>>> >
>>> > Is that necessarily an improvement? I know it hides the cast, but I
>>> > think having an explicit pgd_t* to pud_t* cast that so obviously
>>> > applies to CONFIG_PGTABLE_LEVELS < 4 only is fine as well.
>>>
>>> True; it's not a big thing either way.
>>
>> Sorry,  I'm gonig to change my mind on that again. I think using
>> p?d_offset_kimg is preferable. e.g.
>>
>> static inline pud_t * fixmap_pud(unsigned long addr)
>> {
>>         pgd_t *pgd = pgd_offset_k(addr);
>>
>>         BUG_ON(pgd_none(*pgd) || pgd_bad(*pgd));
>>
>>         return pud_offset_kimg(pgd, addr);
>> }
>>
>> static inline pmd_t * fixmap_pmd(unsigned long addr)
>> {
>>         pud_t *pud = fixmap_pud(addr);
>>
>>         BUG_ON(pud_none(*pud) || pud_bad(*pud));
>>
>>         return pmd_offset_kimg(pud, addr);
>> }
>>
>> That avoids having to check CONFIG_PGTABLE_LEVELS check and perform a cast,
>> avoids duplicating details about bm_{pud,pmd}, and keeps the existing structure
>> so it's easier to reason about the change. I was wrong about having to pass the
>> pgd or pud in, so callers don't need upating.
>>
>> From my PoV that is preferable.
>>
>
> OK. I think it looks better, indeed.

... however, this does mean we have to go through a __pa() translation
and back just to get to the address of bm_pud/bm_pmd