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

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Jan 26, 2018 at 05:34:28PM +0000, Michael Kelley (EOSG) wrote:
> > -----Original Message-----
> > From: Greg KH [mailto:gregkh@linuxfoundation.org]
> > Sent: Thursday, January 25, 2018 2:00 AM
> > To: Michael Kelley (EOSG) <Michael.H.Kelley@microsoft.com>
> > Cc: linux-kernel@vger.kernel.org; devel@linuxdriverproject.org; olaf@aepfle.de;
> > apw@canonical.com; vkuznets@redhat.com; jasowang@redhat.com;
> > leann.ogasawara@canonical.com; marcelo.cerri@canonical.com; Stephen Hemminger
> > <sthemmin@microsoft.com>; KY Srinivasan <kys@microsoft.com>; x86@kernel.org;
> > jrp@pun.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@exchange.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
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