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

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Message-ID: <20200717085442.GX10769@hirez.programming.kicks-ass.net>
Date:   Fri, 17 Jul 2020 10:54:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     ira.weiny@...el.com
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 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.