Date: Wed, 22 Mar 2006 09:27:11 -0800
From: Chris Wright <chrisw@sous-sol.org>
To: Anthony Liguori <aliguori@us.ibm.com>
Cc: Chris Wright <chrisw@sous-sol.org>, linux-kernel@vger.kernel.org,
       virtualization@lists.osdl.org, xen-devel@lists.xensource.com
Subject: Re: [RFC PATCH 00/35] Xen i386 paravirtualization support
Message-ID: <20060322172711.GW15997@sorel.sous-sol.org>
References: <20060322063040.960068000@sorel.sous-sol.org> <4421863C.4070403@us.ibm.com>
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* Anthony Liguori (aliguori@us.ibm.com) wrote:
> Chris Wright wrote:
> >Xen also provides support for running directly on native hardware.
> 
> Can someone elaborate on this?  Does this mean a Xen guest can run on 
> bare metal?

Yes.  See the Xen code for running the kernel in ring0 with Xen
(supervisor_mode_kenel).  The hypercall_page is conditionally filled
with hypercall traps or direct calls basically.

> Is there code available to make this work (it doesn't seem contained in 
> this patchset)?  Has any performance analysis been done?

I don't have any numbers.

> The numbers that have been posted with the VMI patches suggest that some 
> rather tricky stuff is required to achieve native performance when 
> running a guest on bare metal.  If this is not the case, it would be 
> very interesting to know because it seems to be the hairiest part of the 
> VMI patches.

It is a hairy part of VMI.  They've done a nice job of handling the
native case, and have interseting plans for improving the non-native
case (inline where possible).  One of the differences is things that
don't actually require hypercalls are already inline w/ Xen.  So it's
conceivable that the performance hit is smaller than what VMI found
without carefully inlining native code.

> Otherwise, if we want to support Xen guests on bare metal, it seems we 
> would have to change things in the subarch code a bit to do something 
> similar to VMI.

It's a different approach.

thanks,
-chris
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