Re: [RFC PATCH v7 17/23] kernel/entry: Add support for core-wide protection of kernel-mode

[Dario Faggioli]

On Wed, 2020-09-02 at 09:53 +0200, Thomas Gleixner wrote:
> On Tue, Sep 01 2020 at 21:29, Joel Fernandes wrote:
> > On Tue, Sep 01, 2020 at 10:02:10PM +0200, Thomas Gleixner wrote:
> > > 
> > Or, are you saying users may want 'core scheduling' enabled but may
> > want to
> > leave out the kernel protection?
> 
> Core scheduling per se without all the protection muck, i.e. a
> relaxed
> version which tries to gang schedule threads of a process on a core
> if
> feasible has advantages to some workloads.
> 
Indeed! For at least two reasons, IMO:

1) what Thomas is saying already. I.e., even on a CPU which has HT but
is not affected by any of the (known!) speculation issues, one may want
to use Core Scheduling _as_a_feature_. For instance, for avoiding
threads from different processes, or vCPUs from different VMs, sharing
cores (e.g., for better managing their behavior/performance, or for
improved fairness of billing/accounting). And in this case, this
mechanism for protecting the kernel from the userspace on the other
thread may not be necessary or interesting;

2) protection of the kernel from the other thread running in userspace
may be achieved in different ways. This is one, sure. ASI will probably
be another. Hence if/when we'll have both, this and ASI, it would be
cool to be able to configure the system in such a way that there is
only one active, to avoid paying the price of both! :-)

Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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