linux-kernel - Re: [PATCH V7 3/6] arm64/perf: Add branch stack support in struct arm

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Message-ID: <656cdc72-b01c-914e-9ace-59cc9fea572f@arm.com>
Date:   Thu, 9 Feb 2023 09:10:50 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH V7 3/6] arm64/perf: Add branch stack support in struct
 arm_pmu



On 2/9/23 00:56, Mark Rutland wrote:
> On Fri, Jan 13, 2023 at 09:45:22AM +0530, Anshuman Khandual wrote:
>>
>> On 1/12/23 19:24, Mark Rutland wrote:
>>> On Thu, Jan 05, 2023 at 08:40:36AM +0530, Anshuman Khandual wrote:
>>>>  struct arm_pmu {
>>>>  	struct pmu	pmu;
>>>>  	cpumask_t	supported_cpus;
>>>>  	char		*name;
>>>>  	int		pmuver;
>>>> +	int		features;
>>>>  	irqreturn_t	(*handle_irq)(struct arm_pmu *pmu);
>>>>  	void		(*enable)(struct perf_event *event);
>>>>  	void		(*disable)(struct perf_event *event);
>>>
>>> Hmm, we already have the secure_access field separately. How about we fold that
>>> in and go with:
>>>
>>> 	unsigned int	secure_access    : 1,
>>> 			has_branch_stack : 1;
>>
>> Something like this would work, but should we use __u32 instead of unsigned int
>> to ensure 32 bit width ?
> 
> I don't think that's necessary; the exact size doesn't really matter, and
> unsigned int is 32-bits on all targets suppropted by Linux, not just arm and
> arm64.
> 
> I do agree that if this were a userspace ABI detail, it might be preferable to
> use __u32. However, I think using it here gives the misleading impression that
> there is an ABI concern when there is not, and as above it's not necessary, so
> I'd prefer unsigned int here.

Makes sense, will this as unsigned int.