From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755535AbeAHS0e (ORCPT + 1 other); Mon, 8 Jan 2018 13:26:34 -0500 Received: from wtarreau.pck.nerim.net ([62.212.114.60]:38986 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755047AbeAHS0c (ORCPT ); Mon, 8 Jan 2018 13:26:32 -0500 Date: Mon, 8 Jan 2018 19:26:22 +0100 From: Willy Tarreau To: Yves-Alexis Perez Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: Feedback on 4.9 performance after PTI fixes Message-ID: <20180108182622.GM10913@1wt.eu> References: <20180107101856.GA9590@1wt.eu> <1515431274.3305.59.camel@debian.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1515431274.3305.59.camel@debian.org> User-Agent: Mutt/1.6.1 (2016-04-27) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: Hi Yves-Alexis, On Mon, Jan 08, 2018 at 06:07:54PM +0100, Yves-Alexis Perez wrote: > On Sun, 2018-01-07 at 11:18 +0100, Willy Tarreau wrote: > > - the highest performance impact on VMs comes from having PTI on the > > guest kernel (-45%). At this point it makes no difference whether > > the host kernel has it or not. > > Hi Willy, > > out of curiosity, is the pcid/invpcid flags exposed to and used by your guest > CPU? It might very well that the PCID optimisations are not used by the guests > here, and it might be worth either checking on bare metal or with the PCID > optimisations enabled. You're totally right, I discovered during my later developments that indeed PCID is not exposed there. So we take the hit of a full TLB flush twice per syscall. Willy