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

[Scott Wood <scottwood@freescale.com>]

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?

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.

> > 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.

-Scott