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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 05/01/15 15:41, Ian Campbell wrote:
> 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)

This is going to cause problems with the effort to unify PVH and HVM
into a single guest type, which is why I raised the point.  The Linux
PVH code is already using it as a PVH vs non-PVH flag.

Either way - these are not problems which need fixing right now, but do
need to be considered as the unification work progresses.

>
>> 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...

Considering the implications of running dom0 in ring0, pvops seems like
a much more sensible idea.

~Andrew