linux-kernel - Re: page table isolation alternative mechanism

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Message-ID: <20180103201526.3e8251c6@alans-desktop>
Date:   Wed, 3 Jan 2018 20:15:26 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Albert Cahalan <acahalan@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: page table isolation alternative mechanism

On Wed, 3 Jan 2018 14:22:37 -0500
Albert Cahalan <acahalan@...il.com> wrote:

> 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.

I can see how that works for 32bit assuming you don't set up the fast
syscall/ret stuff but for 64bit I don't see how you'd make it work easily
because a syscall isn't an interrupt or trap any more so it can't be a
task or any other kind of gate.

Alan