linux-kernel - Re: [PATCH RFC V2 02/17] x86/fpu: Refactor arch_set_user_pkey

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Date:   Mon, 20 Jul 2020 11:13:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     ira.weiny@...el.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC V2 02/17] x86/fpu: Refactor
 arch_set_user_pkey_access() for PKS support

On Fri, Jul 17, 2020 at 03:36:12PM -0700, Dave Hansen wrote:
> On 7/17/20 1:54 AM, Peter Zijlstra wrote:
> > This is unbelievable junk...
> 
> Ouch!
> 
> This is from the original user pkeys implementation.

The thing I fell over most was new in this patch; the naming of that
function. It doesn't 'get' anything, nor does it allocate anything, so
'new' is out the window too.

> > How about something like:
> > 
> > u32 update_pkey_reg(u32 pk_reg, int pkey, unsigned int flags)
> > {
> > 	int pkey_shift = pkey * PKR_BITS_PER_PKEY;
> > 
> > 	pk_reg &= ~(((1 << PKR_BITS_PER_PKEY) - 1) << pkey_shift);
> > 
> > 	if (flags & PKEY_DISABLE_ACCESS)
> > 		pk_reg |= PKR_AD_BIT << pkey_shift;
> > 	if (flags & PKEY_DISABLE_WRITE)
> > 		pk_reg |= PKR_WD_BIT << pkey_shift;
> > 
> > 	return pk_reg;
> > }
> > 
> > Then we at least have a little clue wtf the thing does.. Yes I started
> > with a rename and then got annoyed at the implementation too.
> 
> That's fine, if some comments get added.

I'm not sure what you would want commented; the code is trivial.

> It looks correct to me but
> probably compiles down to pretty much the same thing as what was there.
>  FWIW, I prefer the explicit masking off of two bit values to implicit
> masking off with a mask generated from PKR_BITS_PER_PKEY.  It's
> certainly more compact, but I usually don't fret over the lines of code.

This way you're sure there are no bits missed. Both the shift and mask
use the same value.