linux-kernel - page table isolation alternative mechanism

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Message-ID: <CABfYdSoeZm-r5aYN7=aJY5+TcaKEA+4AxfBCk_sM=NNsrAp-ug@mail.gmail.com>
Date:   Wed, 3 Jan 2018 14:22:37 -0500
From:   Albert Cahalan <acahalan@...il.com>
To:     linux-kernel@...r.kernel.org
Subject: page table isolation alternative mechanism

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.