linux-kernel - Re: [RFC 03/23] ARM: LPAE: use phys_addr

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Message-ID: <20120724110254.GG20132@arm.com>
Date:	Tue, 24 Jul 2012 12:02:54 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Cyril Chemparathy <cyril@...com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nico@...aro.org" <nico@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	Vitaly Andrianov <vitalya@...com>
Subject: Re: [RFC 03/23] ARM: LPAE: use phys_addr_t on virt <--> phys
 conversion

On Tue, Jul 24, 2012 at 11:55:30AM +0100, Cyril Chemparathy wrote:
> On 7/24/2012 6:37 AM, Catalin Marinas wrote:
> > On Tue, Jul 24, 2012 at 02:09:05AM +0100, Cyril Chemparathy wrote:
> >> This patch fixes up the types used when converting back and forth between
> >> physical and virtual addresses.
> >>
> >> Signed-off-by: Vitaly Andrianov <vitalya@...com>
> >> Signed-off-by: Cyril Chemparathy <cyril@...com>
> >> ---
> >>   arch/arm/include/asm/memory.h |   17 +++++++++++------
> >>   1 file changed, 11 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/arch/arm/include/asm/memory.h b/arch/arm/include/asm/memory.h
> >> index fcb5757..7629dfe 100644
> >> --- a/arch/arm/include/asm/memory.h
> >> +++ b/arch/arm/include/asm/memory.h
> >> @@ -169,22 +169,27 @@ extern unsigned long __pv_phys_offset;
> >>   	: "=r" (to)					\
> >>   	: "r" (from), "I" (type))
> >>
> >> -static inline unsigned long __virt_to_phys(unsigned long x)
> >> +static inline phys_addr_t __virt_to_phys(unsigned long x)
> >>   {
> >>   	unsigned long t;
> >>   	__pv_stub(x, t, "add", __PV_BITS_31_24);
> >>   	return t;
> >>   }
> >>
> >> -static inline unsigned long __phys_to_virt(unsigned long x)
> >> +static inline unsigned long __phys_to_virt(phys_addr_t x)
> >>   {
> >>   	unsigned long t;
> >>   	__pv_stub(x, t, "sub", __PV_BITS_31_24);
> >>   	return t;
> >>   }
> >
> > BTW, I would prefer if the phys-to-virt patching was fixed as well. It
> > shouldn't be difficult.
> 
> On that topic - yes, we have this on our radar, but in our case we;re 
> talking about patching (in head.S) and then repatching (at switch over).
> 
> One of the ideas we've been bouncing around has been to convert the 
> phys-virt patch code into arithmetic on PFNs.  This way we don't have to 
> get too messy with 64-bit in the patch code, and we can use the same 
> patch code for both phys_to_virt and virt_to_phys.  Thoughts on this 
> approach?

It may be slightly less efficient with Thumb-2 code as we can't have add
and lsl in the same instruction. It may anyway be faster than reading a
global variable.

But I suspect you can just mask out the top 32-bit part of a physical
address when converting to virtual. The opposite may also work, you just
have some constant in the top 32-bit part of the phys address. Note that
virt_to_phys is only valid for the lowmem, so 32-bit arithmetic with
some constant for the top 32-bit should be enough.

-- 
Catalin
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