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

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Date:   Fri, 17 Jul 2020 13:52:55 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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 10:54:42AM +0200, Peter Zijlstra wrote:
> On Fri, Jul 17, 2020 at 12:20:41AM -0700, ira.weiny@...el.com wrote:
> > +/*
> > + * Get a new pkey register value from the user values specified.
> > + *
> > + * Kernel users use the same flags as user space:
> > + *     PKEY_DISABLE_ACCESS
> > + *     PKEY_DISABLE_WRITE
> > + */
> > +u32 get_new_pkr(u32 old_pkr, int pkey, unsigned long init_val)
> > +{
> > +	int pkey_shift = (pkey * PKR_BITS_PER_PKEY);
> > +	u32 new_pkr_bits = 0;
> > +
> > +	/* Set the bits we need in the register:  */
> > +	if (init_val & PKEY_DISABLE_ACCESS)
> > +		new_pkr_bits |= PKR_AD_BIT;
> > +	if (init_val & PKEY_DISABLE_WRITE)
> > +		new_pkr_bits |= PKR_WD_BIT;
> > +
> > +	/* Shift the bits in to the correct place: */
> > +	new_pkr_bits <<= pkey_shift;
> > +
> > +	/* Mask off any old bits in place: */
> > +	old_pkr &= ~((PKR_AD_BIT | PKR_WD_BIT) << pkey_shift);
> > +
> > +	/* Return the old part along with the new part: */
> > +	return old_pkr | new_pkr_bits;
> > +}
> 
> This is unbelievable junk...
> 
> 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.

On the code I think this is fair.  I've also updated the calling function to be
a bit cleaner as well.

However, I think the name 'update' is a bit misleading.  Here is the new
calling code:

...
        pkru = read_pkru();
	pkru = update_pkey_reg(pkru, pkey, init_val);
	write_pkru(pkru);
...


I think it is odd to have a function called update_pkey_reg() called right
before a write_pkru().  Can we call this update_pkey_value?  or just 'val'?
Because write_pkru() actually updates the register.

Thanks for the review,
Ira