linux-kernel - Re: [PATCH v2 02/13] arm64: introduce KIMAGE_VADDR as the virtual base of the kernel region

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Date:	Tue, 5 Jan 2016 14:46:50 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Christoffer Dall <christoffer.dall@...aro.org>
Cc:	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	kernel-hardening@...ts.openwall.com, will.deacon@....com,
	catalin.marinas@....com, leif.lindholm@...aro.org,
	keescook@...omium.org, linux-kernel@...r.kernel.org,
	stuart.yoder@...escale.com, bhupesh.sharma@...escale.com,
	arnd@...db.de, marc.zyngier@....com
Subject: Re: [PATCH v2 02/13] arm64: introduce KIMAGE_VADDR as the virtual
 base of the kernel region

On Tue, Jan 05, 2016 at 03:36:34PM +0100, Christoffer Dall wrote:
> On Wed, Dec 30, 2015 at 04:26:01PM +0100, Ard Biesheuvel wrote:
> > This introduces the preprocessor symbol KIMAGE_VADDR which will serve as
> > the symbolic virtual base of the kernel region, i.e., the kernel's virtual
> > offset will be KIMAGE_VADDR + TEXT_OFFSET. For now, we define it as being
> > equal to PAGE_OFFSET, but in the future, it will be moved below it once
> > we move the kernel virtual mapping out of the linear mapping.
> > 
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> > ---
> >  arch/arm64/include/asm/memory.h | 10 ++++++++--
> >  arch/arm64/kernel/head.S        |  2 +-
> >  arch/arm64/kernel/vmlinux.lds.S |  4 ++--
> >  3 files changed, 11 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> > index 853953cd1f08..bea9631b34a8 100644
> > --- a/arch/arm64/include/asm/memory.h
> > +++ b/arch/arm64/include/asm/memory.h
> > @@ -51,7 +51,8 @@
> >  #define VA_BITS			(CONFIG_ARM64_VA_BITS)
> >  #define VA_START		(UL(0xffffffffffffffff) << VA_BITS)
> >  #define PAGE_OFFSET		(UL(0xffffffffffffffff) << (VA_BITS - 1))
> > -#define MODULES_END		(PAGE_OFFSET)
> > +#define KIMAGE_VADDR		(PAGE_OFFSET)
> > +#define MODULES_END		(KIMAGE_VADDR)
> >  #define MODULES_VADDR		(MODULES_END - SZ_64M)
> >  #define PCI_IO_END		(MODULES_VADDR - SZ_2M)
> >  #define PCI_IO_START		(PCI_IO_END - PCI_IO_SIZE)
> > @@ -75,8 +76,13 @@
> >   * private definitions which should NOT be used outside memory.h
> >   * files.  Use virt_to_phys/phys_to_virt/__pa/__va instead.
> >   */
> > -#define __virt_to_phys(x)	(((phys_addr_t)(x) - PAGE_OFFSET + PHYS_OFFSET))
> > +#define __virt_to_phys(x) ({						\
> > +	phys_addr_t __x = (phys_addr_t)(x);				\
> > +	__x >= PAGE_OFFSET ? (__x - PAGE_OFFSET + PHYS_OFFSET) :	\
> > +			     (__x - KIMAGE_VADDR + PHYS_OFFSET); })
> 
> so __virt_to_phys will now work with a subset of the non-linear namely
> all except vmalloced and ioremapped ones?

It will work for linear mapped memory and for the kernel image, which is
what it used to do. It's just that the relationship between the image
and the linear map is broken.

The same rules apply to x86, where their virt_to_phys eventually boils down to:

static inline unsigned long __phys_addr_nodebug(unsigned long x)
{
        unsigned long y = x - __START_KERNEL_map;

        /* use the carry flag to determine if x was < __START_KERNEL_map */
        x = y + ((x > y) ? phys_base : (__START_KERNEL_map - PAGE_OFFSET));

        return x;
}

Thanks,
Mark.
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