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

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Date:   Wed, 22 Nov 2017 19:37:19 +0000
From:   Will Deacon <will.deacon@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 00/18] arm64: Unmap the kernel whilst running in
 userspace (KAISER)

On Mon, Nov 20, 2017 at 06:20:39PM +0000, Ard Biesheuvel wrote:
> On 20 November 2017 at 18:06, Will Deacon <will.deacon@....com> 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.

Will