Re: [Xen-devel] [PATCH] Always save/restore performance counters when HVM guest switching VCPU

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

> > > >>>This was on Intel.
> > > >>That's a really hefty expense to make all users pay on every context
> > > >>switch, on behalf of a random check in a piece of software that only a
> > > >>handful of people are going to be actually using.
> > > >I believe Linux uses perf infrastructure to implement the watchdog.


And as mentioned here, this never gets enabled by default b/c to use
the perf infrastructure you need 'vpmu=1' on the bootup line.

> > 
> > And by default it won't work as for Intel you need these flags:
> > 
> > cpuid=['0xa:eax=0x07300403,ebx=0x00000004,ecx=0x00000000,edx=0x00000603' ]
> 
> This cpuid config variable should not be needed if your cpu is supported in
> vmx_vpmu_initialise() where you added a lot of processors with your patch.
> If not supported and you should see a message in the xen logs.

Found out that is b/c I was using 'xend'. With that there are some
CPUID flags cleared while 'xl' does not do it. 
> 
> > 
> > What we get right now when booting PVHVM under Intel is:
> > 
> > [    0.160989] Performance Events: unsupported p6 CPU model 45 no PMU 
> > driver, software events only.
> > [    0.168098] NMI watchdog disabled (cpu0): hardware events not enabled
> 
> Did you add vpmu to the xen boot parameter list?

Yes.
> 
> I installed opensuse-12.2 as a HVM guest with xen-unstable running and the 
> kernel
> log says:

With Xen 4.3 and using 'xl' I get it working too. Albeit on that
particular box (model 45) I am hitting some weird crashes while on
on a lesser SandyBridge (model 44) it works OK. Anyhow that is
a different thread.

> 
> Mar  7 15:06:18 linux kernel: [    0.183217] CPU0: Intel(R) Core(TM)2 Duo CPU 
>     P8800  @ 2.66GHz stepping 0a
> Mar  7 15:06:18 linux kernel: [    0.183980] Performance Events: 4-deep LBR, 
> Core2 events, Intel PMU driver.
> Mar  7 15:06:18 linux kernel: [    0.189994] ... version:                2
> Mar  7 15:06:18 linux kernel: [    0.189997] ... bit width:              40
> Mar  7 15:06:18 linux kernel: [    0.190000] ... generic registers:      2
> Mar  7 15:06:18 linux kernel: [    0.190002] ... value mask:             
> 000000ffffffffff
> Mar  7 15:06:18 linux kernel: [    0.190005] ... max period:             
> 000000007fffffff
> Mar  7 15:06:18 linux kernel: [    0.190008] ... fixed-purpose events:   3
> Mar  7 15:06:18 linux kernel: [    0.190011] ... event mask:             
> 0000000700000003
> Mar  7 15:06:18 linux kernel: [    0.198203] NMI watchdog: enabled, takes one 
> hw-pmu counter.
> 
> When I call perf:
> 
> # perf stat ls
> acpid             cups      kdm.log        mail.err        news              
> wtmp            zypper.log
> alternatives.log  faillog   krb5           mail.info       ntp               
> Xorg.0.log
> boot.log          firewall  lastlog        mail.warn       pm-powersave.log  
> Xorg.0.log.old
> btmp              hp        localmessages  messages        samba             
> YaST2
> ConsoleKit        journal   mail           NetworkManager  warn              
> zypp
> 
>  Performance counter stats for 'ls':
> 
>           7.840869 task-clock                #    0.590 CPUs utilized         
>  
>                 59 context-switches          #    0.008 M/sec                 
>  
>                  0 CPU-migrations            #    0.000 K/sec                 
>  
>                304 page-faults               #    0.039 M/sec                 
>  
>          6,583,834 cycles                    #    0.840 GHz                   
>   [40.38%]
>    <not supported> stalled-cycles-frontend 
>    <not supported> stalled-cycles-backend  
>          2,168,931 instructions              #    0.33  insns per cycle       
>   [73.20%]
>            525,628 branches                  #   67.037 M/sec                 
>   [79.06%]
>             27,138 branch-misses             #    5.16% of all branches       
>   [83.55%]
> 
>        0.013283672 seconds time elapsed
> 
> As you can see performance counters are working for instructions, branches
> and branch-misses.
> 
> When I call this command in the dom0 it's a bit different:
> 
> # perf stat ls
> acpid             journal        messages           wpa_supplicant.log
> alternatives.log  kdm.log        NetworkManager     wtmp
> boot.log          krb5           news               xen
> btmp              lastlog        ntp                Xorg.0.log
> ConsoleKit        localmessages  pk_backend_zypp    Xorg.0.log.old
> cups              mail           pk_backend_zypp-1  YaST2
> faillog           mail.err       pm-powersave.log   zypp
> firewall          mail.info      samba              zypper.log
> hp                mail.warn      warn               zypper.log-20130307.xz
> 
>  Performance counter stats for 'ls':
> 
>           6.959326 task-clock                #    0.714 CPUs utilized         
>  
>                 11 context-switches          #    0.002 M/sec                 
>  
>                  0 CPU-migrations            #    0.000 K/sec                 
>  
>                304 page-faults               #    0.044 M/sec                 
>  
>    <not supported> cycles                  
>    <not supported> stalled-cycles-frontend 
>    <not supported> stalled-cycles-backend  
>    <not supported> instructions            
>    <not supported> branches                
>    <not supported> branch-misses           
> 
>        0.009746152 seconds time elapsed
> 
> This is because the hardware events are not supported in PV.

Right.

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