Re: [PATCH] arm64/hw_breakpoint: Determine lengths from generic perf breakpoint macros

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 2/26/24 16:34, Mark Rutland wrote:
> On Mon, Feb 26, 2024 at 08:19:39AM +0530, Anshuman Khandual wrote:
>> On 2/23/24 18:22, Will Deacon wrote:
>>> On Fri, Feb 23, 2024 at 05:01:02PM +0530, Anshuman Khandual wrote:
>>>> Both platform i.e ARM_BREAKPOINT_LEN_X and generic i.e HW_BREAKPOINT_LEN_X
>>>> macros are used interchangeably to convert event->attr.bp_len and platform
>>>> breakpoint control arch_hw_breakpoint_ctrl->len. Let's be consistent while
>>>> deriving one from the other. This does not cause any functional changes.
>>>>
>>>> Cc: Will Deacon <will@kernel.org>
>>>> Cc: Mark Rutland <mark.rutland@arm.com>
>>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>>> Cc: linux-arm-kernel@lists.infradead.org
>>>> Cc: linux-kernel@vger.kernel.org
>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>>>> ---
>>>> This applies on v6.8-rc5
>>>>
>>>>  arch/arm64/kernel/hw_breakpoint.c | 16 ++++++++--------
>>>>  1 file changed, 8 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
>>>> index 35225632d70a..1ab9fc865ddd 100644
>>>> --- a/arch/arm64/kernel/hw_breakpoint.c
>>>> +++ b/arch/arm64/kernel/hw_breakpoint.c
>>>> @@ -301,28 +301,28 @@ static int get_hbp_len(u8 hbp_len)
>>>>  
>>>>  	switch (hbp_len) {
>>>>  	case ARM_BREAKPOINT_LEN_1:
>>>> -		len_in_bytes = 1;
>>>> +		len_in_bytes = HW_BREAKPOINT_LEN_1;
>>>
>>> I don't think we should do this. The HW_BREAKPOINT_LEN_* definitions are
>>> part of the user ABI and, although they correspond to the length in bytes,
>>> that's not necessarily something we should rely on.
>>
>> Why should not we rely on the user ABI macros if these byte lengths were
>> initially derived from them. 
> 
> Why should we change the clear:
> 	
> 	len_in_bytes = 1;
> 
> ... to the longer, and less clear:
> 
> 	len_in_bytes = HW_BREAKPOINT_LEN_1;
> 
> ... ?
> 
>> But also there are similar conversions in arch_bp_generic_fields().
> 
> Those are specifically for converting from the rch_hw_breakpoint_ctrl encodings
> to the perf_event_attr encodings. There we don't care about the specific value
> of the byte, just that we're using the correct encoding.
> 
>> These hard coded raw byte length numbers seems cryptic, where as in reality
>> these are just inter converted from generic HW breakpoints lengths.
> 
> There are three distinct concepts here:
> 
> 1. The length in bytes, as returned above by get_hbp_len()
> 
> 2. The length as encoded in the ARM_BREAKPOINT_LEN_* encoding
> 
> 3. The length as encoded in the HW_BREAKPOINT_LEN_* encoding.
> 
> I think you're arguing that since 1 and 3 happen to have the values we should
> treat them as the same thing. I think that Will and I believe that they should
> be kept distinct because they are distinct concepts.
> 
> I don't think this needs to change, and can be left as-is.

Fair enough, but just wondering how about deriving len_in_bytes from
hweight_long(ARM_BREAKPOINT_LEN_*) instead ? This also drops the hard
coding using the platform macros itself, without going to user ABI.