linux-kernel - Re: [RFC PATCH 6/7] arm64: KVM: Handle trappable TLB instructions

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Message-ID: <20160901145525.GM6721@arm.com>
Date:   Thu, 1 Sep 2016 15:55:25 +0100
From:   Will Deacon <will.deacon@....com>
To:     Punit Agrawal <punit.agrawal@....com>
Cc:     kvm@...r.kernel.org, Marc Zyngier <marc.zyngier@....com>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 6/7] arm64: KVM: Handle trappable TLB instructions

On Fri, Aug 26, 2016 at 10:37:08AM +0100, Punit Agrawal wrote:
> > Will Deacon <will.deacon@....com> writes:
> >> The easiest thing to do is just TLBI VMALLE1IS for all trapped operations,
> >> but you might want to see how that performs.
> >
> > That sounds reasonable for correctness. But I suspect we'll have to do
> > more to claw back some performance. Let me run a few tests and come back
> > on this.
> 
> Assuming I've correctly switched in TCR and replacing the various TLB
> operations in this patch with TLBI VMALLE1IS, there is a drop in kernel
> build times of ~5% (384s vs 363s).

What do you mean by "switched in TCR"? Why is that necessary if you just
nuke the whole thing? Is the ~5% relative to no trapping at all, or
trapping, but being selective about the operation?

Will