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

Avi Kivity
2010-03-16T09:32:17Z

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.

>>> 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.

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

--
error compiling committee.c: too many arguments to function