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

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Message-ID: <YjC2jcn7kJKdHrf3@iweiny-desk3>
Date:   Tue, 15 Mar 2022 08:53:49 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Dave Hansen <dave.hansen@...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 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);

> 
> Also if we're worried about the type needing to change 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.

Ira