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Subject: Re: [patch V163 27/51] x86/mm/pti: Populate user PGD
To: Peter Zijlstra <peterz@infradead.org>
References: <20171218114215.239543034@linutronix.de>
 <20171218115255.827361623@linutronix.de>
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Cc: Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirsky <luto@kernel.org>, Borislav Petkov <bpetkov@suse.de>,
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        Will Deacon <will.deacon@arm.com>, daniel.gruss@iaik.tugraz.at,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@suse.de>,
        "H. Peter Anvin" <hpa@zytor.com>
From: Dave Hansen <dave.hansen@intel.com>
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Date: Mon, 18 Dec 2017 12:45:13 -0800
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On 12/18/2017 12:41 PM, Peter Zijlstra wrote:
>> I also don't think the user_shared area of the fixmap can get *that*
>> big.  Does anybody know offhand what the theoretical limits are there?
> Problem there is the nr_cpus term I think, we currently have up to 8k
> CPUs, but I can see that getting bigger in the future.

It only matters if we go over 512GB, though.  Is the per-cpu part of the
fixmap ever more than 512GB/8k=64MB?

In any case, it's not hard to make a little loop.  I think the common
case, at least is that we only do 4 PGDs.