On Wed, Mar 14, 2018 at 09:25:17PM +0200, Michael S. Tsirkin 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 :)
Does the hardware notify the kernel that it changes? Or does the kernel have to "poll" for it? > In particular, it changes whenever VM is migrated or snapshotted. So very rarely. And userspace always knows about those events already, right? > > > 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. "every"? That's horrid, why would you write a database that has to do an ACPI i/o call for every transaction? That's a sure way to write a very slow database :( > So I'm pretty sure it's a performance sensitive path. Given that this api is not present today, why is this even needed? Who wants/needs it so badly that it has to be tuned in ways like this? If it is _really_ performant critical, just make it a new syscall :) > But yes, I > didn't profile any apps since they > are yet to be written to use this interface. Then what database are you talking about? What apps need/want this thing? thanks, greg k-h
