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From: Andrew Lutomirski <amluto@gmail.com>
Date: Mon, 21 Apr 2014 18:06:19 -0700
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Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: "H. Peter Anvin" <hpa@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Alexander van Heukelum <heukelum@fastmail.fm>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Borislav Petkov <bp@alien8.de>,
        Arjan van de Ven <arjan.van.de.ven@intel.com>,
        Brian Gerst <brgerst@gmail.com>,
        Alexandre Julliard <julliard@winehq.com>,
        Andi Kleen <andi@firstfloor.org>, Thomas Gleixner <tglx@linutronix.de>
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On Mon, Apr 21, 2014 at 5:53 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> Well, if 2^17 CPUs are allocated we might 2K pages allocated.  We could easily do a bitmap here, of course.  NR_CPUS/64 is a small number, and would reduce the code complexity.
>

Even simpler: just get rid of the check entirely.  That is, break out
of the higher level loops once one of them is set (this should be a
big speedup regardless) and don't allocate the page if the first PTE
is already pointing at something.

After all, espfix_already_there is mostly a duplicate of init_espfix_cpu.

--Andy