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Date:   Mon, 25 Mar 2019 17:38:04 -0700
From:   Andi Kleen <ak@linux.intel.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     "Chang S. Bae" <chang.seok.bae@intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Ravi Shankar <ravi.v.shankar@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>, x86@kernel.org
Subject: Re: [RESEND PATCH v6 04/12] x86/fsgsbase/64: Enable FSGSBASE
 instructions in the helper functions
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>    So on user space to kernel space transitions swapping in kernel GS should
>    simply do:
>      userGS = RDGSBASE()
>      WRGSBASE(kernelGS)

This would also need to find kernelGS first, by doing RDPID and then
reading it  from memory in the right index
(which might be a full cache miss if you're unlucky)

SWAPGS will be a lot faster, especially in these rare worst cases
because it has all its state inside the CPU.

-Andi

BTW you managed to only review after Chang went on a long vacation.

<rant>
I don't understand why it takes that long to review these changes
It's one of the largest performance improvements for the context
switch and the NMI in many years plus gives a new free register
to user space, but it only makes progress at a glacial pace.
The original patches for this were posted in 2016.
</rant>