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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Mar 15, 2018 at 08:57:05AM +0200, Gal Hammer wrote:
> On Wed, Mar 14, 2018 at 9:25 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > 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.
> 
> This value doesn't change when a VM is migrated. It is unlikely that
> this value will be changed so frequently that a direct access to the
> memory is required. Even in QEMU, the current implementation was
> merged without an option to change the generation id on-the-fly. One
> must run a new instance in order to set a new value, which means that
> no application will be running during that time.


The point is still that it changes without an application
or the kernel doing anything.

> >> > 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.
> 
> I agree with Greg here. The user is able to read the value, and then
> wait for a notification if she cares about changes.

OK.  Patch has to implement notifications for this to work though.
That's missing.

> >> 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
> 
> Thanks,
> 
>     Gal.