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
