Re: [PATCH v2 2/2] ARM: support PHYS_OFFSET minimum aligned at 64KiB boundary

[Leizhen (ThunderTown)]

On 2020/9/17 22:00, Ard Biesheuvel wrote:
> On Tue, 15 Sep 2020 at 22:06, Russell King - ARM Linux admin
> <linux@xxxxxxxxxxxxxxx> wrote:
>>
>> 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@xxxxxxxxxx>
>>> ---
>>>  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?
>>
> 
> Since LPAE implies v7, we can use movw unconditionally, which is nice.
> 
> There is no need to use an additional temp register, as we can use the
> register holding the high word. (There is no need for the mov_hi macro
> to be separate)
> 
> 0:     movw    %R0, #low offset >> 16
>        adds    %Q0, %1, %R0, lsl #16
> 1:     mov     %R0, #high offset
>        adc     %R0, %R0, #0
>        .pushsection .pv_table,"a"
>        .long 0b, 1b
>        .popsection
> 
> The only problem is distinguishing the two mov instructions from each

The #high offset can also consider use movw, it just save two bytes in
the thumb2 scenario. We can store different imm16 value for high_offset
and low_offset, so that we can distinguish them in __fixup_a_pv_table().

This will make the final implementation of the code look more clear and
consistent, especially THUMB2.

Let me try it.

> other, but that should not be too hard I think.
> 
> .
>