linux-kernel - Re: [PATCH 2/3] KVM: improve trace events of vmexit/mmio/ioport

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Date:	Wed, 18 Jan 2012 10:32:08 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	David Ahern <dsahern@...il.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, Avi Kivity <avi@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/3] KVM: improve trace events of vmexit/mmio/ioport

On 01/18/2012 01:31 AM, David Ahern wrote:


>>>>
>>>> Relying on the existing tracepoints will make the tool work on older
>>>> kernels.
>>>>
>>>
>>>
>>> We can drop all new events, but unfortunately, the information of the origin
>>> tracepoints is not enough, at least vcpu_id need be traced in theses events
>>> to match its events. Yes?
>>
>> Yes, and if you don't add completed events, you'll have to use kvm_entry 
>> whose collection adds a lot of overhead.
> 
> 
> vcpus run as threads and hence have their own tid which is available in
> the perf samples.
> 


Nice, thanks for your information, David!

> There is an ongoing 'discussion' about modifying existing tracepoints
> which your proposed patch violates. This is the start of it:
> 


Hmm, i think we can still add completed tracepoints in the new version kernel,
if old version kernel is used, we can fall back to use kvm_entry instead?

And there is a exception for mmio read, in current code, the mmio read event is
actually used to trace the time when then read emulation is completed, i think
we can add a tracepoint like mmio_read_begin to trace the start time of mmio read.
So:
- for the new kernel, we use mmio_read_begion and kvm_mmio(READ...) to calculate
  start time and end time.
- for the old kernel, we use kvm_exit and kvm_mmio(READ...) to calculate start time
  and end time.

Your idea?

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