page table isolation alternative mechanism

page table isolation alternative mechanism

[Albert Cahalan]

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.

Re: page table isolation alternative mechanism

[Alan Cox]

On Wed, 3 Jan 2018 14:22:37 -0500
Albert Cahalan <acahalan@gmail.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