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

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Date:   Mon, 11 Feb 2019 10:02:47 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Kimberly Brown <kimbrownkd@...il.com>
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, 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.
 
> No, I'm not working on a project emulating Hyper-V.

OK, I had heard that KVM project was doing something with QEMU.

> >   
> > > +
> > > +	if (!hv_dev->channel->offermsg.monitor_allocated)
> > > +		return sprintf(buf, "\n");  
> > 
> > If monitor is not used, why not return an error instead of empty
> > data. Any program (or user) would have to handle that already.  
> 
> I think that returning an error instead is fine. I'll make this change
> in the next version of the patch.