Re: [PATCHv2 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set

From: Eduardo Valentin <eduval@amazon.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Eduardo Valentin <eduval@amazon.com>, <rkrcmar@redhat.com>,
	Matt Wilson <msw@amazon.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: [PATCHv2 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set
Date: Thu, 2 Nov 2017 11:43:11 -0700	[thread overview]
Message-ID: <20171102184311.GC27676@u40b0340c692b58f6553c.ant.amazon.com> (raw)
In-Reply-To: <39241f1f-1b88-709e-21b1-2019ad156b08@redhat.com>

On Thu, Nov 02, 2017 at 07:24:16PM +0100, Paolo Bonzini wrote:
> On 02/11/2017 19:08, Eduardo Valentin wrote:
> > On Thu, Nov 02, 2017 at 06:56:46PM +0100, Paolo Bonzini wrote:
> >> On 02/11/2017 18:45, Eduardo Valentin wrote:
> >>> Currently, the existing qspinlock implementation will fallback to
> >>> test-and-set if the hypervisor has not set the PV_UNHALT flag.
> >>>
> >>> 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.
> >>
> >> Have you seen Waiman's series that lets you specify this on the guest
> >> command line instead?  Would this be acceptable for your use case?
> > 
> > No, can you please share a link to it? is it already merged to tip/master?
> 
> [PATCH-tip v2 0/2] x86/paravirt: Enable users to choose PV lock type
> https://lkml.org/lkml/2017/11/1/655
> 
> >> (In other words, is there a difference for you between making the host
> >> vs. guest administrator toggle the feature?  "@amazon.com" means you are
> >> the host admin, how would you use it?)
> > 
> > The way I think of this is this is a flag set by host side so the
> > guest adapts accordingly.
> > 
> > If the admin in guest side wants to ignore what the host is
> > flagging, that is a different story.
> 
> Okay, this makes sense.  But perhaps it should be a separate CPUID leaf,
> such as "configuration hints", rather than properly a feature.

Oh OK, you don't think this starts to deviate from the feature concept.
But would the PV_UNHALT also go to "configuration hints" bucket?

Another way to see this is we have three locking feature options to select from,
so we need at least two bits here.

> 
> Paolo

-- 
All the best,
Eduardo Valentin