From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Ahern <dsahern@gmail.com>
Subject: Re: BUG: perf kvm top --callgraph not showing callgraph
Date: Fri, 14 Jun 2013 17:23:20 -0600
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To: Cody P Schafer <devel-lists@codyps.com>
Cc: linux-perf-users@vger.kernel.org, KVM <kvm@vger.kernel.org>, Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>

[added kvm devel list and Xiao who implemented the original perf-kvm as 
I recall]

On 6/14/13 5:11 PM, Cody P Schafer wrote:
>> I am under the impression the limitations are these 2 snippets in
>> >arch/x86/kernel/cpu/perf_event.c:
> Yep, that's disabling the in-kernel callchain generation (the in-perf
> callchain code should still be an option without this). I wonder if
> one could use perf_callchain_user() to find the callchain of the guest
> kernel.
>
> Does anyone know the particular changes that need to be done to
> support kvm backtraces/the reasons it is specifically disabled?

It's going to require some KVM changes I would think. e.g., for the IP:

unsigned long perf_instruction_pointer(struct pt_regs *regs)
{
     if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
         return perf_guest_cbs->get_guest_ip();

     return regs->ip + code_segment_base(regs);
}

get_guest_ip comes from arch/x86/kvm/x86.c.

callchain walking starts in perf_callchain, kernel/events/callchain.c

For more context:
perf_prepare_sample()
   data->callchain = perf_callchain(event, regs);

For guests you'll want regs from the guest which will need to be 
supplied by kvm.

David