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Subject: Re: [PATCH-tip v2 2/2] x86/xen: Deprecate xen_nopvspin
To:     Waiman Long <longman@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Jonathan Corbet <corbet@lwn.net>
References: <1509569934-4919-1-git-send-email-longman@redhat.com>
 <1509569934-4919-3-git-send-email-longman@redhat.com>
Cc:     x86@kernel.org, virtualization@lists.linux-foundation.org,
        kvm@vger.kernel.org, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, Juergen Gross <jgross@suse.com>,
        Alok Kataria <akataria@vmware.com>,
        Rusty Russell <rusty@rustcorp.com.au>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <5c36ed18-e59e-1ea3-28e0-414f88965761@oracle.com>
Date:   Wed, 1 Nov 2017 18:01:10 -0400
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On 11/01/2017 04:58 PM, Waiman Long wrote:
> +/* TODO: To be removed in a future kernel version */
>  static __init int xen_parse_nopvspin(char *arg)
>  {
> -	xen_pvspin = false;
> +	pr_warn("xen_nopvspin is deprecated, replace it with \"pvlock_type=queued\"!\n");
> +	if (!pv_spinlock_type)
> +		pv_spinlock_type = locktype_queued;

Since we currently end up using unfair locks and because you are
deprecating xen_nopvspin I wonder whether it would be better to set this
to locktype_unfair so that current behavior doesn't change. (Sorry, I
haven't responded to your earlier message before you posted this). Juergen?

I am also not sure I agree with making pv_spinlock an enum *and* a
bitmask at the same time. I understand that it makes checks easier but I
think not assuming a value or a pattern would be better, especially
since none of the uses is on a critical path.

(For example, !pv_spinlock_type is the same as locktype_auto, which is
defined but never used)


-boris

>  	return 0;
>  }
>  early_param("xen_nopvspin", xen_parse_nopvspin);
> -


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