Re: [PATCH 3/7] x86/enter: Use IBRS on syscall and interrupts

[Peter Zijlstra]

On Thu, Jan 04, 2018 at 09:56:44AM -0800, Tim Chen wrote:
> Set IBRS upon kernel entrance via syscall and interrupts. Clear it
> upon exit.

So not only did we add a CR3 write, we're now adding an MSR write to the
entry/exit paths. Please tell me that these are 'fast' MSRs? Given
people are already reporting stupid numbers with just the existing
PTI/CR3, what kind of pain are we going to get from adding this?

> If NMI runs when exiting kernel between IBRS_DISABLE and
> SWAPGS, the NMI would have turned on IBRS bit 0 and then it would have
> left enabled when exiting the NMI. IBRS bit 0 would then be left
> enabled in userland until the next enter kernel.
> 
> That is a minor inefficiency only, but we can eliminate it by saving
> the MSR when entering the NMI in save_paranoid and restoring it when
> exiting the NMI.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>

Invalid SoB chain, either you lost a From: Andrea or you need something
else.