linux-kernel - Re: [PATCH v5] drivers/misc: vm_gen_counter: initial driver implementation

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Date:   Wed, 14 Mar 2018 19:25:36 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Or Idgar <idgar@...tualoco.com>, linux-kernel@...r.kernel.org,
        arnd@...db.de, oidgar@...hat.com, ghammer@...hat.com,
        Or Idgar <oridgar@...il.com>
Subject: Re: [PATCH v5] drivers/misc: vm_gen_counter: initial driver
 implementation

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?

> 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