[PATCH 1/4] ARM: runtime patching of __virt_to_phys() and __phys_to_virt()
Russell King - ARM Linux
linux at arm.linux.org.uk
Tue Jan 4 03:45:17 EST 2011
This is basically my patch with a few blank lines removed, a couple
of \n's also removed, a #error if __virt_to_phys is defined by a platform,
a minor tweak to the assembly and it being only usable on PXA.
I much prefer my patch over this as anyone can use it. That's one of
the reasons why I arranged the code testing for __virt_to_phys as I
did, so the config option could be offered without having a big long
dependency list attached to it.
On Tue, Jan 04, 2011 at 03:20:05AM -0500, Nicolas Pitre wrote:
> On ARM it is common to find different offsets for the location of
> physical memory. In order to support multiple machine types with
> a single kernel binary, we need to make PHYS_OFFSET a variable.
> But turning PHYS_OFFSET into a global variable would impact performance
> of many hot paths.
>
> In the context of __virt_to_phys() and __phys_to_virt(), we currently have:
>
> #define __virt_to_phys(x) ((x) - PAGE_OFFSET + PHYS_OFFSET)
>
> This normally translates into the following assembly instruction:
>
> add rx, rx, #(PHYS_OFFSET - PAGE_OFFSET)
>
> If we can assume that the difference between PHYS_OFFSET and PAGE_OFFSET
> will always fit into 8 bits shifted to the MSBs, then we can easily patch
> this difference into the corresponding assembly instructions at run time.
> This is like saying that phys and virt offsets will always be at least
> 16 MB aligned which is a pretty safe assumption.
>
> So the idea is to create a table of pointers to all those add instructions,
> and have the early boot code to walk and patch them up before the kernel
> gets to use them. Result is equivalent to a variable PHYS_OFFSET with
> next to zero performance impact compared to the constant PHYS_OFFSET.
>
> Right now, the difference between PHYS_OFFSET and PAGE_OFFSET is determined
> by the actual physical address the kernel is executing from upon start,
> assuming that the kernel is located within the first 16 MB of RAM.
>
> Thanks to Eric Miao and Russell King for their contributions to this patch.
>
> Signed-off-by: Nicolas Pitre <nicolas.pitre at linaro.org>
> ---
> arch/arm/Kconfig | 5 ++++
> arch/arm/include/asm/memory.h | 51 +++++++++++++++++++++++++++++++++--------
> arch/arm/kernel/head.S | 35 ++++++++++++++++++++++++++++
> arch/arm/kernel/vmlinux.lds.S | 4 +++
> 4 files changed, 85 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index d56d21c0..136ed9b 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -187,6 +187,11 @@ config VECTORS_BASE
> help
> The base address of exception vectors.
>
> +config ARM_PATCH_PHYS_VIRT
> + bool
> + depends on EXPERIMENTAL
> + depends on !XIP && !THUMB2_KERNEL
> +
> source "init/Kconfig"
>
> source "kernel/Kconfig.freezer"
> diff --git a/arch/arm/include/asm/memory.h b/arch/arm/include/asm/memory.h
> index 23c2e8e..2783ce2 100644
> --- a/arch/arm/include/asm/memory.h
> +++ b/arch/arm/include/asm/memory.h
> @@ -133,16 +133,6 @@
> #endif
>
> /*
> - * Physical vs virtual RAM address space conversion. These are
> - * private definitions which should NOT be used outside memory.h
> - * files. Use virt_to_phys/phys_to_virt/__pa/__va instead.
> - */
> -#ifndef __virt_to_phys
> -#define __virt_to_phys(x) ((x) - PAGE_OFFSET + PHYS_OFFSET)
> -#define __phys_to_virt(x) ((x) - PHYS_OFFSET + PAGE_OFFSET)
> -#endif
> -
> -/*
> * Convert a physical address to a Page Frame Number and back
> */
> #define __phys_to_pfn(paddr) ((paddr) >> PAGE_SHIFT)
> @@ -157,6 +147,47 @@
> #ifndef __ASSEMBLY__
>
> /*
> + * Physical vs virtual RAM address space conversion. These are
> + * private definitions which should NOT be used outside memory.h
> + * files. Use virt_to_phys/phys_to_virt/__pa/__va instead.
> + */
> +#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
> +
> +#ifdef __virt_to_phys
> +#error "this machine configuration uses complex __virt_to_phys/__phys_to_virt and cannot use CONFIG_ARM_PATCH_PHYS_VIRT"
> +#endif
> +
> +#define __pv_stub(from,to,instr) \
> + __asm__( \
> + "1: " instr "\t%0, %1, %2\n" \
> + " .pushsection .pv_table,\"a\"\n" \
> + " .long 1b\n" \
> + " .popsection" \
> + : "=r" (to) \
> + : "r" (from), "I" (1))
> +
> +static inline unsigned long __virt_to_phys(unsigned long x)
> +{
> + unsigned long t;
> +
> + __pv_stub(x, t, "add");
> + return t;
> +}
> +
> +static inline unsigned long __phys_to_virt(unsigned long x)
> +{
> + unsigned long t;
> +
> + __pv_stub(x, t, "sub");
> + return t;
> +}
> +
> +#else
> +#define __virt_to_phys(x) ((x) - PAGE_OFFSET + PHYS_OFFSET)
> +#define __phys_to_virt(x) ((x) - PHYS_OFFSET + PAGE_OFFSET)
> +#endif
> +
> +/*
> * The DMA mask corresponding to the maximum bus address allocatable
> * using GFP_DMA. The default here places no restriction on DMA
> * allocations. This must be the smallest DMA mask in the system,
> diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
> index 6bd82d2..eaaf0ad 100644
> --- a/arch/arm/kernel/head.S
> +++ b/arch/arm/kernel/head.S
> @@ -95,6 +95,9 @@ ENTRY(stext)
> #ifdef CONFIG_SMP_ON_UP
> bl __fixup_smp
> #endif
> +#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
> + bl __fixup_pv_table
> +#endif
> bl __create_page_tables
>
> /*
> @@ -433,4 +436,36 @@ smp_on_up:
>
> #endif
>
> +#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
> +
> +/* __fixup_pv_table - patch the stub instructions with the delta between
> + * PHYS_OFFSET and PAGE_OFFSET, which is assumed to be 16MiB aligned and
> + * can be expressed by an immediate shifter operand. The stub instruction
> + * has a form of '(add|sub) rd, rn, #imm'.
> + */
> +__fixup_pv_table:
> + adr r0, 1f
> + ldmia r0, {r3-r5}
> + sub r3, r0, r3 @ PHYS_OFFSET - PAGE_OFFSET
> + mov r6, r3, lsr #24 @ constant for add/sub instructions
> + teq r3, r6, lsl #24 @ must be 16MiB aligned
> + bne __error
> + orr r6, r6, #0x400 @ mask in rotate right 8 bits
> + add r4, r4, r3
> + add r5, r5, r3
> +2: cmp r4, r5
> + ldrlo r7, [r4], #4
> + ldrlo ip, [r7, r3]
> + mov ip, ip, lsr #12
> + orr ip, r6, ip, lsl #12
> + strlo ip, [r7, r3]
> + blo 2b
> + mov pc, lr
> +ENDPROC(__fixup_phys_virt)
> +
> +1: .word .
> + .word __pv_table_begin
> + .word __pv_table_end
> +#endif
> +
> #include "head-common.S"
> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> index cead889..fb32c9d 100644
> --- a/arch/arm/kernel/vmlinux.lds.S
> +++ b/arch/arm/kernel/vmlinux.lds.S
> @@ -57,6 +57,10 @@ SECTIONS
> __smpalt_end = .;
> #endif
>
> + __pv_table_begin = .;
> + *(.pv_table)
> + __pv_table_end = .;
> +
> INIT_SETUP(16)
>
> INIT_CALLS
> --
> 1.7.3.2.193.g78bbb
>
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