[PATCH 2/2] KVM: PPC: Book3E: Emulate MCSRR0/1 SPR and rfmci instruction

[Alexander Graf]

On 10.07.2013, at 20:24, Scott Wood wrote:

> On 07/10/2013 05:23:36 AM, Alexander Graf wrote:
>> On 10.07.2013, at 00:26, Scott Wood wrote:
>> > On 07/09/2013 05:00:26 PM, Alexander Graf wrote:
>> >> It'll also be more flexible at the same time. You could take the logs and actually check what's going on to debug issues that you're encountering for example.
>> >> We could even go as far as sharing the same tool with other architectures, so that we only have to learn how to debug things once.
>> >
>> > Have you encountered an actual need for this flexibility, or is it theoretical?
>> Yeah, first thing I did back then to actually debug kvm failures was to add trace points.
> 
> I meant specifically for handling exit timings this way.

No, but I did encounter the need for debugging exits. And the only thing we would need to get exit timing stats once we already have trace points for exit types would be to have trace points for guest entry and maybe type specific events to indicate what the exit is about as well.

> 
>> > Is there common infrastructure for dealing with measuring intervals and tracking statistics thereof, rather than just tracking points and letting userspace connect the dots (though it could still do that as an option)?  Even if it must be done in userspace, it doesn't seem like something that should be KVM-specific.
>> Would you like to have different ways of measuring mm subsystem overhead? I don't :). The same goes for KVM really. If we could converge towards a single user space interface to get exit timings, it'd make debugging a lot easier.
> 
> I agree -- that's why I said it doesn't seem like something that should be KVM-specific.  But that's orthogonal to whether it's done in kernel space or user space.  The ability to get begin/end events from userspace would be nice when it is specifically requested, but it would also be nice if the kernel could track some basic statistics so we wouldn't have to ship so much data around to arrive at the same result.
> 
> At the very least, I'd like such a tool/infrastructure to exist before we start complaining about doing minor maintenance of the current mechanism.

I admit that I don't fully understand qemu/scripts/kvm/kvm_stat, but it seems to me as if it already does pretty much what we want. It sets up a filter to only get events and their time stamps through.

It does use normal exit trace points on x86 to replace the old debugfs based stat counters. And it seems to work reasonably well for that.

> 
>> We already have this for the debugfs counters btw. And the timing framework does break kvm_stat today already, as it emits textual stats rather than numbers which all of the other debugfs stats do. But at least I can take the x86 kvm_stat tool and run it on ppc just fine to see exit stats.
> 
> We already have what?  The last two sentences seem contradictory -- can you or can't you use kvm_stat as is?  I'm not familiar with kvm_stat.

Kvm_stat back in the day used debugfs to give you an idea on what exit event happens most often. That mechanism got replaced by trace points later which the current kvm_stat uses.

I still have a copy of the old kvm_stat that I always use to get a first feeling for what goes wrong if something goes wrong. The original code couldn't deal with the fact that we have a debugfs file that contains text though. I patched it locally. It also works just fine if you simply disable timing stats, since then you won't have the text file.

> What does x86 KVM expose in debugfs?

The same thing it always exposed - exit stats. I am fairly sure Avi wanted to completely deprecate that interface in favor of the trace point based approach, but I don't think he ever got around to it.


Alex