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

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Message-ID: <20160111172100.GA6105@leverpostej>
Date:	Mon, 11 Jan 2016 17:21:01 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
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 Mon, Jan 11, 2016 at 06:15:56PM +0100, Ard Biesheuvel wrote:
> 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:
> >> 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

True, but we only do it in the case of a one-off init function, so I
don't think we'll notice the overhead.

Mark.