linux-kernel - Re: [PATCH RFC 0/3] Add guest cpu

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Message-ID: <503BEC0D.9090409@redhat.com>
Date:	Mon, 27 Aug 2012 14:52:13 -0700
From:	Avi Kivity <avi@...hat.com>
To:	mjw@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	peterz@...radead.org, mtosatti@...hat.com, glommer@...allels.com,
	mingo@...hat.com
Subject: Re: [PATCH RFC 0/3] Add guest cpu_entitlement reporting

On 08/27/2012 02:27 PM, Michael Wolf wrote:
> On Mon, 2012-08-27 at 13:31 -0700, Avi Kivity wrote:
> > On 08/27/2012 01:23 PM, Michael Wolf wrote:
> > > > 
> > > > How would a guest know what its entitlement is?
> > > > 
> > > > 
> > >
> > > Currently the Admin/management tool setting up the guests will put it on
> > > the qemu commandline.  From this it is passed via an ioctl to the host.
> > > The guest will get the value from the host via a hypercall.
> > >
> > > In the future the host could try and do some of it automatically in some
> > > cases. 
> > 
> > Seems to me it's a meaningless value for the guest.  Suppose it is
> > migrated to a host that is more powerful, and as a result its relative
> > entitlement is reduced.  The value needs to be adjusted.
>
> This is why I chose to manage the value from the sysctl interface rather
> than just have it stored as a value in /proc.  Whatever tool was used to
> migrate the vm could hopefully adjust the sysctl value on the guest.

We usually try to avoid this type of coupling.  What if the guest is
rebooting while this is happening?  What if it's not running Linux at all?

> > 
> > This is best taken care of from the host side.
>
> Not sure what you are getting at here.  If you are running in a cloud
> environment, you purchase a VM with the understanding that you are
> getting certain resources.  As this type of user I don't believe you
> have any access to the host to see this type of information.  So the
> user still wouldnt have a way to confirm that they are receiving what
> they should be in the way of processor resources.
>
> Would you please elaborate a little more on this?

I meant not reporting this time as steal time.  But that cripples steal
time reporting.

Looks like for each quanta we need to report how much real time has
passed, how much the guest was actually using, and how much the guest
was not using due to overcommit (with the reminder being unallocated
time).  The guest could then present it any way it wanted to.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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