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Subject: Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls
References: <20160707124719.3F04C882@viggo.jf.intel.com>
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 <20160708071810.GA27457@gmail.com> <577FD587.6050101@sr71.net>
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From: Dave Hansen <dave@sr71.net>
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Date: Tue, 12 Jul 2016 08:39:07 -0700
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To: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>, linux-arch <linux-arch@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>, "linux-mm@kvack.org" <linux-mm@kvack.org>, Mel Gorman <mgorman@techsingularity.net>, Linus Torvalds <torvalds@linux-foundation.org>, Andrew Morton <akpm@linux-foundation.org>, Linux API <linux-api@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Al Viro <viro@zeniv.linux.org.uk>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Hugh Dickins <hughd@google.com>, "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>, Dave Hansen <dave.hansen@linux.intel.com>

On 07/12/2016 12:13 AM, Ingo Molnar wrote:
>> > Remember, PKRU is just a *bitmap*.  The only place keys are stored is in the 
>> > page tables.
> A pkey is an index *and* a protection mask. So by representing it as a bitmask we 
> lose per thread information. This is what I meant by 'incomplete shadowing' - for 
> example the debug code couldn't work: if we cleared a pkey in a task we wouldn't 
> know what to restore it to with the current data structures, right?

Right.  I actually have some code to do the shadowing that I wrote to
explore how to do different PKRU values in signal handlers.  The code
only shadowed the keys that were currently allocated, and used the
(mm-wide) allocation map to figure that out.  It did not have a separate
per-thread concept of which parts of PKRU need to be shadowed.

It essentially populated the shadow value on all pkru_set() calls.

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