MIME-Version: 1.0
From: Albert Cahalan <acahalan@gmail.com>
Date: Wed, 3 Jan 2018 14:22:37 -0500
Message-ID: <CABfYdSoeZm-r5aYN7=aJY5+TcaKEA+4AxfBCk_sM=NNsrAp-ug@mail.gmail.com>
Subject: page table isolation alternative mechanism
To: linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org

We got into the current situation for performance reasons, avoiding the costly
reload of CR3 that a hardware task switch would cause. It seems we'll be
loading CR3 now anyway, so it might be time to reconsider hardware
task switches.

The recent patches leave kernel entry/exit code mapped. Hardware task switches
wouldn't need that. All they need is a single entry in a reduced-size
IDT, for the
doublefault, and a minimal GDT, and a TSS. Taking the fault switches CR3. That
then gets you a proper IDT and GDT because those are virtually mapped.
Not a single byte of kernel code would need to be mapped while user code runs.