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SubjectRe: [PATCH v2 2/2] ARM: support PHYS_OFFSET minimum aligned at 64KiB boundary
On Tue, Sep 15, 2020 at 09:16:15PM +0800, Zhen Lei wrote:
> Currently, only support the kernels where the base of physical memory is
> at a 16MiB boundary. Because the add/sub instructions only contains 8bits
> unrotated value. But we can use one more "add/sub" instructions to handle
> bits 23-16. The performance will be slightly affected.
>
> Since most boards meet 16 MiB alignment, so add a new configuration
> option ARM_PATCH_PHYS_VIRT_RADICAL (default n) to control it. Say Y if
> anyone really needs it.
>
> All r0-r7 (r1 = machine no, r2 = atags or dtb, in the start-up phase) are
> used in __fixup_a_pv_table() now, but the callee saved r11 is not used in
> the whole head.S file. So choose it.
>
> Because the calculation of "y = x + __pv_offset[63:24]" have been done,
> so we only need to calculate "y = y + __pv_offset[23:16]", that's why
> the parameters "to" and "from" of __pv_stub() and __pv_add_carry_stub()
> in the scope of CONFIG_ARM_PATCH_PHYS_VIRT_RADICAL are all passed "t"
> (above y).
>
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
> arch/arm/Kconfig | 18 +++++++++++++++++-
> arch/arm/include/asm/memory.h | 16 +++++++++++++---
> arch/arm/kernel/head.S | 25 +++++++++++++++++++------
> 3 files changed, 49 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index e00d94b16658765..19fc2c746e2ce29 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -240,12 +240,28 @@ config ARM_PATCH_PHYS_VIRT
> kernel in system memory.
>
> This can only be used with non-XIP MMU kernels where the base
> - of physical memory is at a 16MB boundary.
> + of physical memory is at a 16MiB boundary.
>
> Only disable this option if you know that you do not require
> this feature (eg, building a kernel for a single machine) and
> you need to shrink the kernel to the minimal size.
>
> +config ARM_PATCH_PHYS_VIRT_RADICAL
> + bool "Support PHYS_OFFSET minimum aligned at 64KiB boundary"
> + default n

Please drop the "default n" - this is the default anyway.

> @@ -236,6 +243,9 @@ static inline unsigned long __phys_to_virt(phys_addr_t x)
> * in place where 'r' 32 bit operand is expected.
> */
> __pv_stub((unsigned long) x, t, "sub", __PV_BITS_31_24);
> +#ifdef CONFIG_ARM_PATCH_PHYS_VIRT_RADICAL
> + __pv_stub((unsigned long) t, t, "sub", __PV_BITS_23_16);

t is already unsigned long, so this cast is not necessary.

I've been debating whether it would be better to use "movw" for this
for ARMv7. In other words:

movw tmp, #16-bit
adds %Q0, %1, tmp, lsl #16
adc %R0, %R0, #0

It would certainly be less instructions, but at the cost of an
additional register - and we'd have to change the fixup code to
know about movw.

Thoughts?

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