linux-kernel - Re: [PATCH] kvm: log directly from the guest to the host kvm buffer

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Message-ID: <4DCBFFF2.3030804@redhat.com>
Date:	Thu, 12 May 2011 18:42:42 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Dhaval Giani <dhaval.giani@...il.com>
CC:	kvm@...r.kernel.org, joro@...tes.org, agraf@...e.de,
	rostedt@...dmis.org, linux-kernel@...r.kernel.org,
	Fabio Checconi <fchecconi@...il.com>,
	Tommaso Cucinotta <tommaso.cucinotta@...up.it>
Subject: Re: [PATCH] kvm: log directly from the guest to the host kvm buffer

On 05/12/2011 06:39 PM, Dhaval Giani wrote:
> >
> >  I think that one hypercall per trace is too expensive.  Tracing is meant to
> >  be lightweight!  I think the guest can log to a buffer, which is flushed on
> >  overflow or when a vmexit occurs.  That gives us automatic serialization
> >  between a vcpu and the cpu it runs on, but not between a vcpu and a
> >  different host cpu.
> >
>
> hmm. So, basically, log all of these events, and then send them to the
> host either on an exit, or when your buffer fills up. There is one
> problem with approach though. One of the reasons I wanted this
> approach was beacuse i wanted to co-relate the guest and the host
> times. (which is why I kept is synchronous). I lose out that
> information with what you say. However I see your point about the
> overhead. I will think about this a bit more.

You might use kvmclock to get a zero-exit (but not zero-cost) time which 
can be correlated.

Another option is to use xadd on a shared memory area to have a global 
counter incremented.  However that can be slow on large machines, and is 
hard to do securely with multiple guests.


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

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