Re: [PATCH xen-4.6] xen: Remove CONFIG_X86_SUPERVISOR_MODE_KERNEL as x86_32 builds are unsupported

[Ian Campbell <Ian.Campbell@citrix.com>]

On Mon, 2015-01-05 at 15:35 +0000, Andrew Cooper wrote:
> Answering out-of-order
> 
> This patch has no functional change WRT PVH.  The hunk commented on is
> simply changed via indentation due to the removal of the conditional it
> is in.  It was never been possible for a PVH kernel boot with
> "!XENFEAT_supervisor_mode_kernel" in its feature string.

Oh, good!

> If retcon'ing this feature flag is acceptable then the problem goes
> away, as can the commented hunk.  However, given the meaning of the
> flag, it really should be advertised to plain HVM guests as well,
> because their kernel does run in ring0.  At that point, it becomes more
> of a general "running in an hvm container" flag.

Conversely one could argue that it is a PV* only feature which makes no
sense to an HVM guest (which I think is approximately what it means now)

> What usecase was supervisor_mode_kernel developed for?  It seems
> counter-intuitive, but I can't find anything in the history explaining
> its use.

It was a prototype from the pre-pvops days to see if it would be
feasible to have a single kernel binary which ran either on Xen or on a
stub hypervisor which ran it "as native" with little or no loss of
performance^TM (e.g. for distro's convenience to avoid the multiple
kernel issue).

It never went beyond a prototype with Xen proper instead of the proposed
stub hypervisor and then pvops came along and was a much more sensible
idea...

Ian.