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Date:   Thu, 14 Feb 2019 11:50:19 -0800
From:   Stephen Hemminger <stephen@networkplumber.org>
To:     Kimberly Brown <kimbrownkd@gmail.com>
Cc:     Michael Kelley <mikelley@microsoft.com>,
        Long Li <longli@microsoft.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>,
        Dexuan Cui <decui@microsoft.com>, devel@linuxdriverproject.org,
        Haiyang Zhang <haiyangz@microsoft.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] Drivers: hv: vmbus: Display nothing in sysfs if
 monitor_allocated not set
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On Thu, 14 Feb 2019 01:11:03 -0500
Kimberly Brown <kimbrownkd@gmail.com> wrote:

> 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@gmail.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@gmail.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?

Thanks for following up. I agree with you EINVAL works as a solution.
My understanding is that their are two ways a channel can work. The first one is
for the guest to send a hyper call to the host to indicate when data is available.
The other is for the guest to indicate by setting a bit in shared memory with host.

The shared memory approach reduces host/guest overhead and allows for more opportunities
for batching in the ring. The host checks the shared memory on a polling interval
defined in the latency field.

The hypercall method does not use the monitor page. It has lower latency (no polling).