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From: Andy Lutomirski <luto@kernel.org>
Date: Tue, 20 Mar 2018 15:05:26 +0000
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Subject: Re: [PATCH 00/15] x86: Enable FSGSBASE instructions
To: "Chang S. Bae" <chang.seok.bae@intel.com>
Cc: X86 ML <x86@kernel.org>, Andrew Lutomirski <luto@kernel.org>,
        Andi Kleen <ak@linux.intel.com>, "H. Peter Anvin" <hpa@zytor.com>,
        "Metzger, Markus T" <markus.t.metzger@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Mar 19, 2018 at 5:49 PM, Chang S. Bae <chang.seok.bae@intel.com> wrote:
> FSGSBASE is 64-bit instruction set to allow read/write
> FS/GS base from any privilege. As introduced from
> Ivybridge, enabling effort has been revolving quite long
> [2,3,4] for various reasons. After extended discussions [1],
> this patchset is proposed to introduce new ABIs of
> customizing FS/GS base (separate from its selector).
>
> FSGSBASE-enabled VM can be located on hosts with
> either HW virtualization or SW emulation. KVM advertises
> FSGSBASE when physical CPU has and emulation is
> supported in QEMU/TCG [5]. In a pool of mixed systems, VMM
> may disable FSGSBASE for seamless VM migrations [6].
>
> A couple of major benefits are expected. Kernel will have
> performance improvement in context switch by skipping MSR
> write for GS base. User-level programs (such as JAVA-based)
> benefit from avoiding system calls to edit FS/GS base.

Can you describe what changed since the last submission?  It looks
like a lot has changed and this series is much more complicated and
much more fragile than it used to be.  Why?