linux-kernel - Re: [PATCH 5/5] x86/pkeys: Standardize on u8 for pkey type

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Date:   Tue, 15 Mar 2022 09:03:26 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] x86/pkeys: Standardize on u8 for pkey type

On 3/15/22 08:53, Ira Weiny wrote:
> On Mon, Mar 14, 2022 at 04:49:12PM -0700, Dave Hansen wrote:
>> On 3/10/22 16:57, ira.weiny@...el.com wrote:
>>> From: Ira Weiny <ira.weiny@...el.com>
>>>
>>> The number of pkeys supported on x86 and powerpc are much smaller than a
>>> u16 value can hold.  It is desirable to standardize on the type for
>>> pkeys.  powerpc currently supports the most pkeys at 32.  u8 is plenty
>>> large for that.
>>>
>>> Standardize on the pkey types by changing u16 to u8.
>>
>> How widely was this intended to "standardize" things?  Looks like it may
>> have missed a few spots.
> 
> Sorry I think the commit message is misleading you.  The justification of u8 as
> the proper type is that no arch has a need for more than 255 pkeys.
> 
> This specific patch was intended to only change x86.  Per that goal I don't see
> any other places in x86 which uses u16 after this patch.
> 
> $ git grep u16 arch/x86 | grep key
> arch/x86/events/intel/uncore_discovery.c:	const u16 *type_id = key;
> arch/x86/include/asm/intel_pconfig.h:	u16 keyid;
> arch/x86/include/asm/mmu.h:	u16 pkey_allocation_map;
> arch/x86/include/asm/pkeys.h:	u16 all_pkeys_mask = ((1U << arch_max_pkey()) - 1);

I was also looking at the generic mm code.

>> Also if we're worried about the type needing to changY or with the wrong
>> type being used, I guess we could just to a pkey_t typedef.
> 
> I'm not 'worried' about it.  But I do think it makes the code cleaner and more
> self documenting.

Yeah, consistency is good.  Do you mind taking a look at how a pkey_t
would look, and also seeing how much core mm code should use it?