Re: [PATCH 00/18] arm64: Unmap the kernel whilst running in userspace (KAISER)

From: Will Deacon
Date: Wed Nov 22 2017 - 14:37:26 EST

On Mon, Nov 20, 2017 at 06:20:39PM +0000, Ard Biesheuvel wrote:
> On 20 November 2017 at 18:06, Will Deacon <will.deacon@xxxxxxx> wrote:
> > I'll see if I can measure the cost of the current vbar switching to get
> > an idea of the potential performance available.
> >
> Yeah, makes sense. If the bulk of the performance hit is elsewhere,
> there's no point in focusing on this bit.

I had a go at implementing a variant on your suggestion where we avoid
swizzling the vbar on exception entry/exit but I couldn't reliably measure a
difference in performance. It appears that the ISB needed by the TTBR change
is dominant, so the vbar write is insignificant.