linux-kernel - Re: [PATCH 2/2] Drivers: hv: vmbus: Display nothing in sysfs if monitor

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Date:   Thu, 14 Feb 2019 01:11:03 -0500
From:   Kimberly Brown <kimbrownkd@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Michael Kelley <mikelley@...rosoft.com>,
        Long Li <longli@...rosoft.com>,
        Sasha Levin <Alexander.Levin@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>, devel@...uxdriverproject.org,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] Drivers: hv: vmbus: Display nothing in sysfs if
 monitor_allocated not set

On Mon, Feb 11, 2019 at 10:02:47AM -0800, Stephen Hemminger wrote:
> On Mon, 11 Feb 2019 02:01:18 -0500
> Kimberly Brown <kimbrownkd@...il.com> wrote:
> 
> > On Fri, Feb 08, 2019 at 02:32:09PM -0800, Stephen Hemminger wrote:
> > > On Fri, 8 Feb 2019 05:01:12 -0500
> > > Kimberly Brown <kimbrownkd@...il.com> wrote:
> > > 
> > > You are right, the current behavior is broken.
> > > It would be good to add a description of under what conditions
> > > monitor is not used. Is this some part of a project emulating
> > > Hyper-V?
> > >   
> > 
> > I'm not sure which conditions determine whether the monitor mechanism is
> > used. I've searched the Hypervisor TLFS, and I couldn't find any
> > information. If you have any suggestions for where I can find this
> > information, please let me know.
> 
> The monitor page stuff pre-dates my involvement with Hyper-V. KY might know.
> But based on comments it looks like it was added to avoid hypercalls
> for each message. It probably showed up in Windows Server 2012 timeframe.
> 
> To test you might want to dig up Windows Server 2008.
>  

It looks like the monitor mechanism has always been used. It's present in the
earliest commit that I can find: 3e7ee4902fe6 ("add the Hyper-V virtual bus")
from 2009.

I propose that the following sentences be added to the sysfs documentation for
the affected attributes:

"The monitor page mechanism is used for performance critical channels (storage,
network, etc.). Channels that do not use the monitor page mechanism will return
EINVAL."

I think that this provides sufficient information for a user to understand why
opening an affected file can return EINVAL. What do you think?