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

[Alexander Graf]

On 10.07.2013, at 00:26, Scott Wood wrote:

> On 07/09/2013 05:00:26 PM, Alexander Graf wrote:
>> On 09.07.2013, at 23:54, Scott Wood wrote:
>> > On 07/09/2013 04:49:32 PM, Alexander Graf wrote:
>> >> Not sure I understand. What the timing stats do is that they measure the time between [exit ... entry], right? We'd do the same thing, just all in C code. That means we would become slightly less accurate, but gain dynamic enabling of the traces and get rid of all the timing stat asm code.
>> >
>> > Compile-time enabling bothers me less than a loss of accuracy (not just a small loss by moving into C code, but a potential for a large loss if we overflow the buffer)
>> Then don't overflow the buffer. Make it large enough.
> 
> How large is that?  Does the tool recognize and report when overflow happens?
> 
> How much will the overhead of running some python script on the host, consuming a large volume of data, affect the results?
> 
>> IIRC ftrace improved recently to dynamically increase the buffer size too.
>> Steven, do I remember correctly here?
> 
> Yay more complexity.
> 
> So now we get to worry about possible memory allocations happening when we try to log something?  Or if there is a way to do an "atomic" log, we're back to the "buffer might be full" situation.
> 
>> > and a dependency on a userspace tool
>> We already have that for kvm_stat. It's a simple python script - and you surely have python on your rootfs, no?
>> > (both in terms of the tool needing to be written, and in the hassle of ensuring that it's present in the root filesystem of whatever system I'm testing).  And the whole mechanism will be more complicated.
>> 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.

> 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.

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.

> 
>> > Lots of debug options are enabled at build time; why must this be different?
>> Because I think it's valuable as debug tool for cases where compile time switches are not the best way of debugging things. It's not a high profile thing to tackle for me tbh, but I don't really think working heavily on the timing stat thing is the correct path to walk along.
> 
> Adding new exit types isn't "working heavily" on it.

No, but the fact that the first patch is a fix to add exit stats for exits that we missed out before doesn't give me a lot of confidence that lots of people use timing stats. And I am always very weary of #ifdef'ed code, as it blows up the test matrix heavily.


Alex