Re: [Xen-devel] [PATCH RFC 00/10] x86 passthrough code cleanup

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Sat, Feb 24, 2018 at 03:23:48AM +0000, Tian, Kevin wrote:
> > From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> > Sent: Saturday, February 24, 2018 12:08 AM
> > 
> > On Fri, Feb 23, 2018 at 05:12:05AM +0000, Tian, Kevin wrote:
> > > > From: Wei Liu
> > > > Sent: Thursday, February 22, 2018 5:47 AM
> > > >
> > > > Hi all
> > > >
> > > > At some point I would like to make CONFIG_HVM and CONFIG_PV work.
> > > > The
> > > > passthrough code is one of the road blocks for that work.
> > >
> > > Can you elaborate the motivation of this change? why does someone
> > > want to disable HVM or PV logic completely from hypervisor?
> > >
> > 
> > At some point in the future, we would like to utilise as many hardware
> > features as possible and have an HVM / PVH only setup. At that point PV
> > code will be necessarily and should be preferably compiled out to reduce
> > code size and attack surface.
> > 
> > Having PV compiled out also enable Xen to reclaim some address space
> > from the PV ABI.
> > 
> > But, we understand that PV is here to stay for at least a while and
> > could be useful for some other niche use cases, so upstream have
> > developed a PV-in-PVH shim to continue to support PV guests. The shim is
> > actually yet another configuration of Xen running as a PVH guest but
> > exposes PV ABI to PV guests. We want to disable HVM code in that case,
> > again, to reduce code size and attach surface.
> 
> since it's only temporary, do we really care about code size or attacking
> surface in such case? or is it worthy of the effort regarding to the gain
> which you anticipate?
> 

It depends. We don't really know how long the shim is going to live. If
it is going to be useful for the next 5 to 10 years after we decide to
go HVM / PVH only (which I think that's a reasonable assumption for
enterprise usage), it is worthy of the effort.

> I sort of buy-in CONFIG_PV, but not sure whether CONFIG_HVM is
> required.
> 

I kinda agree: CONFIG_PV is more valuable than CONFIG_HVM. But I have
already done the heavy-lifting to distinguish PV and HVM code so having
CONFIG_HVM work can be seen as a side effect of that.

Wei.

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