Re: [Xen-devel] [PATCH 1/7] arm: traps: psci: use generic register accessors

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 09/08/17 10:52, Julien Grall wrote:
>
>
> On 08/08/17 21:37, Andrew Cooper wrote:
>> On 08/08/2017 21:08, Volodymyr Babchuk wrote:
>>> diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
>>> index 6cf9ee7..ed78b36 100644
>>> --- a/xen/arch/arm/traps.c
>>> +++ b/xen/arch/arm/traps.c
>>> @@ -1449,13 +1449,12 @@ static void do_debug_trap(struct
>>> cpu_user_regs *regs, unsigned int code)
>>>  }
>>>  #endif
>>>
>>> +#define PSCI_SET_RESULT(reg, val) set_user_reg(reg, 0, val)
>>> +#define PSCI_ARG(reg,n) get_user_reg(reg, n)
>>> +
>>>  #ifdef CONFIG_ARM_64
>>> -#define PSCI_RESULT_REG(reg) (reg)->x0
>>> -#define PSCI_ARG(reg,n) (reg)->x##n
>>> -#define PSCI_ARG32(reg,n) (uint32_t)( (reg)->x##n &
>>> 0x00000000FFFFFFFF )
>>> +#define PSCI_ARG32(reg,n) (uint32_t)(get_user_reg(reg, n) &
>>> 0x00000000FFFFFFFF)
>>
>> There is no need for the mask as well as the explicit (uint32_t) cast.
>> I'd recommend dropping the mask entirely.
>
> I want to avoid the implicit cast from 64-bit register to 32-bit that
> Volodymyr introduced in his series.
>
> uint32_t pstate = get_user_reg(regs, 1);

This is how we'd expect code to look on the x86 side, but you are the
maintainer here, so have prerogative.

>
> IHMO this is a call to mistake. Another solution is to provide 3 helpers
>     - get_user_reg32(...)
>     - get_user_reg64(...)
>     - get_user_reg(...) -> Return the full register (32-bit on ARM32,
> 64-bit on ARM64).
>
> This would at least document the return value of get_user_reg*.

None of this is an explanation for having both an explicit uint32_t cast
and mask hidden inside PSCI_ARG32().  They are redundant.

~Andrew

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