linux-kernel - Re: [PATCH 23/30] x86, kaiser: use PCID feature to make user and kernel switches faster

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Date:   Thu, 16 Nov 2017 20:19:31 +0100
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        moritz.lipp@...k.tugraz.at, daniel.gruss@...k.tugraz.at,
        michael.schwarz@...k.tugraz.at, richard.fellner@...dent.tugraz.at,
        luto@...nel.org, torvalds@...ux-foundation.org,
        keescook@...gle.com, hughd@...gle.com, x86@...nel.org
Subject: Re: [PATCH 23/30] x86, kaiser: use PCID feature to make user and
 kernel switches faster

Hello,

On Fri, Nov 10, 2017 at 11:31:50AM -0800, Dave Hansen wrote:
> Hugh Dickins also points out that PCIDs really have two distinct
> use-cases in the context of KAISER.  The first way they can be used

I don't see why you try to retain such a minor optimization for newer
Intel chips when at the same you prevent KAISER to run with good
performance on older Intel chips like SandyBridge/IvyBridge which
would create a major performance regression for those two. I'd prefer
if you reverse the PCID feature of v4.14 when KASIER is enabled (at
build time would be enough initially), and you use just two asids to
only accelerate enter/exit kernel and you flush the whole TLB over mm
switch like Hugh suggested. It may not even be worth to flush over
cr4, as you've only two asids to deal with anyway.