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Date:   Wed, 14 Mar 2018 21:25:17 +0200
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Or Idgar <idgar@virtualoco.com>, linux-kernel@vger.kernel.org,
        arnd@arndb.de, oidgar@redhat.com, ghammer@redhat.com,
        Or Idgar <oridgar@gmail.com>
Subject: Re: [PATCH v5] drivers/misc: vm_gen_counter: initial driver
 implementation
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 <20180314182536.GA14504@kroah.com>
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On Wed, Mar 14, 2018 at 07:25:36PM +0100, Greg KH wrote:
> On Tue, Mar 13, 2018 at 07:40:51PM +0200, Michael S. Tsirkin wrote:
> > I think it's a good idea to use sysfs for this. However,
> > there are a couple of missing interfaces here:
> > 
> > 1. Userspace needs a way to know when this value changes.
> >    I see no change notifications here and that does not seem right.
> 
> How can these change?

It's a hardware register. It changes when hardware feels like it :)
In particular, it changes whenever VM is migrated or snapshotted.

> > 2. Userspace needs to be able to read these without
> >    system calls.
> 
> Ick, what?  Why not?
>
> > Pls add mmap support to the raw format.
> 
> For a single integer?  Why do you need mmap for this?  What is so
> "performant" that needs to touch a sysfs file?
> >    (Phys address is not guaranteed to be page-aligned so you will
> >     probably want an offset attribute for that as well).
> 
> Ick ick ick, that's why it's good to just stick with a sysfs file.
> 
> Have you tested just how long this takes to see if the open/read/close
> is really the bottleneck, or if the io on reading the value is the
> bottleneck?
> 
> thanks,
> 
> greg k-h

Well an application needs to check this value basically after
every database transaction. So I'm pretty sure it's a performance
sensitive path. But yes, I didn't profile any apps since they
are yet to be written to use this interface.
I'm fine deferring point 2 for now.

-- 
MST