From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S932394AbdJXLTK (ORCPT <rfc822;w@1wt.eu>);
        Tue, 24 Oct 2017 07:19:10 -0400
Received: from mx1.redhat.com ([209.132.183.28]:40240 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S932230AbdJXLTH (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 24 Oct 2017 07:19:07 -0400
DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com C9C3A552C7
Authentication-Results: ext-mx05.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com
Authentication-Results: ext-mx05.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=rkrcmar@redhat.com
Date: Tue, 24 Oct 2017 13:18:59 +0200
From: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
To: Eduardo Valentin <eduval@amazon.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Waiman Long <longman@redhat.com>, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        "Jan H . Schoenherr" <jschoenh@amazon.de>,
        Anthony Liguori <aliguori@amazon.com>
Subject: Re: [PATCH 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set
Message-ID: <20171024111859.GA13316@flask>
References: <1508805867-14583-1-git-send-email-eduval@amazon.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1508805867-14583-1-git-send-email-eduval@amazon.com>
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Tue, 24 Oct 2017 11:19:07 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

2017-10-23 17:44-0700, Eduardo Valentin:
> Currently, the existing qspinlock implementation will fallback to
> test-and-set if the hypervisor has not set the PV_UNHALT flag.

Where have you detected the main source of overhead with pinned VCPUs?
Makes me wonder if we couldn't improve general PV_UNHALT,

thanks.

> This patch gives the opportunity to guest kernels to select
> between test-and-set and the regular queueu fair lock implementation
> based on the PV_DEDICATED KVM feature flag. When the PV_DEDICATED
> flag is not set, the code will still fall back to test-and-set,
> but when the PV_DEDICATED flag is set, the code will use
> the regular queue spinlock implementation.

Some flag makes sense and we do want to make sure that userspaces don't
enable it in pass-through-cpuid mode.