Re: [PATCH] Enhance perf to collect KVM guest os statistics from host side

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

On Tue, 2010-03-16 at 11:32 +0200, Avi Kivity wrote:
> On 03/16/2010 09:48 AM, Zhang, Yanmin wrote:
> >
> >> Excellent, support for guest kernel != host kernel is critical (I can't
> >> remember the last time I ran same kernels).
> >>
> >> How would we support multiple guests with different kernels?
> >>      
> > With the patch, 'perf kvm report --sort pid" could show
> > summary statistics for all guest os instances. Then, use
> > parameter --pid of 'perf kvm record' to collect single problematic instance data.
> >    
> 
> That certainly works, though automatic association of guest data with 
> guest symbols is friendlier.
Thanks. Originally, I planed to add a -G parameter to perf. Such like
-G 8888:/XXX/XXX/guestkallsyms:/XXX/XXX/modules,8889:/XXX/XXX/guestkallsyms:/XXX/XXX/modules
8888 and 8889 are just qemu guest pid.

So we could define multiple guest os symbol files. But it seems ugly,
and 'perf kvm report --sort pid" and 'perf kvm top --pid' could provide
similar functionality.

> 
> >>> diff -Nraup linux-2.6_tipmaster0315/arch/x86/kvm/vmx.c linux-2.6_tipmaster0315_perfkvm/arch/x86/kvm/vmx.c
> >>> --- linux-2.6_tipmaster0315/arch/x86/kvm/vmx.c	2010-03-16 08:59:11.825295404 +0800
> >>> +++ linux-2.6_tipmaster0315_perfkvm/arch/x86/kvm/vmx.c	2010-03-16 09:01:09.976084492 +0800
> >>> @@ -26,6 +26,7 @@
> >>>    #include<linux/sched.h>
> >>>    #include<linux/moduleparam.h>
> >>>    #include<linux/ftrace_event.h>
> >>> +#include<linux/perf_event.h>
> >>>    #include "kvm_cache_regs.h"
> >>>    #include "x86.h"
> >>>
> >>> @@ -3632,6 +3633,43 @@ static void update_cr8_intercept(struct
> >>>    	vmcs_write32(TPR_THRESHOLD, irr);
> >>>    }
> >>>
> >>> +DEFINE_PER_CPU(int, kvm_in_guest) = {0};
> >>> +
> >>> +static void kvm_set_in_guest(void)
> >>> +{
> >>> +	percpu_write(kvm_in_guest, 1);
> >>> +}
> >>> +
> >>> +static int kvm_is_in_guest(void)
> >>> +{
> >>> +	return percpu_read(kvm_in_guest);
> >>> +}
> >>>
> >>>        
> >>      
> >    
> >> There is already PF_VCPU for this.
> >>      
> > Right, but there is a scope between kvm_guest_enter and really running
> > in guest os, where a perf event might overflow. Anyway, the scope is very
> > narrow, I will change it to use flag PF_VCPU.
> >    
> 
> There is also a window between setting the flag and calling 'int $2' 
> where an NMI might happen and be accounted incorrectly.
> 
> Perhaps separate the 'int $2' into a direct call into perf and another 
> call for the rest of NMI handling.  I don't see how it would work on svm 
> though - AFAICT the NMI is held whereas vmx swallows it. 

>  I guess NMIs 
> will be disabled until the next IRET so it isn't racy, just tricky.
I'm not sure if vmexit does break NMI context or not. Hardware NMI context
isn't reentrant till a IRET. YangSheng would like to double check it.

> 
> >>> +static struct perf_guest_info_callbacks kvm_guest_cbs = {
> >>> +	.is_in_guest 		= kvm_is_in_guest,
> >>> +	.is_user_mode		= kvm_is_user_mode,
> >>> +	.get_guest_ip		= kvm_get_guest_ip,
> >>> +	.reset_in_guest		= kvm_reset_in_guest,
> >>> +};
> >>>
> >>>        
> >> Should be in common code, not vmx specific.
> >>      
> > Right. I discussed with Yangsheng. I will move above data structures and
> > callbacks to file arch/x86/kvm/x86.c, and add get_ip, a new callback to
> > kvm_x86_ops.
> >    
> 
> You will need access to the vcpu pointer (kvm_rip_read() needs it), you 
> can put it in a percpu variable.
We do so now in a new patch.

>   I guess if it's not null, you know 
> you're in a guest, so no need for PF_VCPU.
Good suggestion.

Thanks.