powerpc/powernv: Don't reprogram SLW image on every KVM guest entry/exit

[Michael Ellerman]

On Tue, 2019-02-12 at 00:58:29 UTC, Paul Mackerras wrote:
> Commit 24be85a23d1f ("powerpc/powernv: Clear PECE1 in LPCR via stop-api
> only on Hotplug", 2017-07-21) added two calls to opal_slw_set_reg()
> inside pnv_cpu_offline(), with the aim of changing the LPCR value in
> the SLW image to disable wakeups from the decrementer while a CPU is
> offline.  However, pnv_cpu_offline() gets called each time a secondary
> CPU thread is woken up to participate in running a KVM guest, that is,
> not just when a CPU is offlined.
> 
> Since opal_slw_set_reg() is a very slow operation (with observed
> execution times around 20 milliseconds), this means that an offline
> secondary CPU can often be busy doing the opal_slw_set_reg() call
> when the primary CPU wants to grab all the secondary threads so that
> it can run a KVM guest.  This leads to messages like "KVM: couldn't
> grab CPU n" being printed and guest execution failing.
> 
> There is no need to reprogram the SLW image on every KVM guest entry
> and exit.  So that we do it only when a CPU is really transitioning
> between online and offline, this moves the calls to
> pnv_program_cpu_hotplug_lpcr() into pnv_smp_cpu_kill_self().
> 
> Fixes: 24be85a23d1f ("powerpc/powernv: Clear PECE1 in LPCR via stop-api only on Hotplug")
> Cc: stable at vger.kernel.org # v4.14+
> Signed-off-by: Paul Mackerras <paulus at ozlabs.org>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/19f8a5b5be2898573a5e1dc1db93e8d4

cheers