linux-kernel - Re: [PATCH v2 char-misc 1/1] Drivers: hv: vmbus: Implement Direct Mode for stimer0

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Date:   Fri, 26 Jan 2018 19:15:39 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "apw@...onical.com" <apw@...onical.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "leann.ogasawara@...onical.com" <leann.ogasawara@...onical.com>,
        "marcelo.cerri@...onical.com" <marcelo.cerri@...onical.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        "x86@...nel.org" <x86@...nel.org>, "jrp@....org" <jrp@....org>
Subject: Re: [PATCH v2 char-misc 1/1] Drivers: hv: vmbus: Implement Direct
 Mode for stimer0

On Fri, Jan 26, 2018 at 05:34:28PM +0000, Michael Kelley (EOSG) wrote:
> > -----Original Message-----
> > From: Greg KH [mailto:gregkh@...uxfoundation.org]
> > Sent: Thursday, January 25, 2018 2:00 AM
> > To: Michael Kelley (EOSG) <Michael.H.Kelley@...rosoft.com>
> > Cc: linux-kernel@...r.kernel.org; devel@...uxdriverproject.org; olaf@...fle.de;
> > apw@...onical.com; vkuznets@...hat.com; jasowang@...hat.com;
> > leann.ogasawara@...onical.com; marcelo.cerri@...onical.com; Stephen Hemminger
> > <sthemmin@...rosoft.com>; KY Srinivasan <kys@...rosoft.com>; x86@...nel.org;
> > jrp@....org
> > Subject: Re: [PATCH v2 char-misc 1/1] Drivers: hv: vmbus: Implement Direct Mode for stimer0
> > 
> > On Mon, Jan 22, 2018 at 03:29:29PM -0700, mikelley@...hange.microsoft.com wrote:
> > > +/*
> > > + * If false, we're using the old mechanism for stimer0 interrupts
> > > + * where it sends a VMbus message when it expires. The old
> > > + * mechanism is used if Direct Mode is explicitly disabled
> > > + * by the module parameter, or on older versions of Hyper-V
> > > + * that don't support Direct Mode. While Hyper-V provides
> > > + * four stimer's per CPU, Linux uses only stimer0.
> > > + */
> > > +static bool direct_mode_enabled = true;
> > > +module_param(direct_mode_enabled, bool, 0444);
> > > +MODULE_PARM_DESC(direct_mode_enabled,
> > > +			"Set to N to disable stimer Direct Mode.");
> > 
> > Ick ick ick.  How is a distro or user supposed to know if/when to enable
> > this and not to enable it?  This isn't ok, can you please make this
> > "automatic" to always do the right thing based on what it is running on?
> 
> To be clear, this patch already does the "automatic" thing based on the
> capabilities of the hypervisor.  But it's often handy to be able to override
> the automatic behavior without having to rebuild the kernel.

Then put the option somewhere else.  sysfs, debugfs, somewhere else.
Never a module parameter please, not for new code.

thanks,

greg k-h