Re: [PATCH 2/2] KVM: PPC: Book3S HV: Implement dynamic micro-threading on POWER8

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, May 28, 2015 at 03:17:20PM +1000, Paul Mackerras wrote:
> This builds on the ability to run more than one vcore on a physical
> core by using the micro-threading (split-core) modes of the POWER8
> chip.  Previously, only vcores from the same VM could be run together,
> and (on POWER8) only if they had just one thread per core.  With the
> ability to split the core on guest entry and unsplit it on guest exit,
> we can run up to 8 vcpu threads from up to 4 different VMs, and we can
> run multiple vcores with 2 or 4 vcpus per vcore.
> 
> Dynamic micro-threading is only available if the static configuration
> of the cores is whole-core mode (unsplit), and only on POWER8.
> 
> To manage this, we introduce a new kvm_split_mode struct which is
> shared across all of the subcores in the core, with a pointer in the
> paca on each thread.  In addition we extend the core_info struct to
> have information on each subcore.  When deciding whether to add a
> vcore to the set already on the core, we now have two possibilities:
> (a) piggyback the vcore onto an existing subcore, or (b) start a new
> subcore.
> 
> Currently, when any vcpu needs to exit the guest and switch to host
> virtual mode, we interrupt all the threads in all subcores and switch
> the core back to whole-core mode.  It may be possible in future to
> allow some of the subcores to keep executing in the guest while
> subcore 0 switches to the host, but that is not implemented in this
> patch.
> 
> This adds a module parameter called dynamic_mt_modes which controls
> which micro-threading (split-core) modes the code will consider, as a
> bitmap.  In other words, if it is 0, no micro-threading mode is
> considered; if it is 2, only 2-way micro-threading is considered; if
> it is 4, only 4-way, and if it is 6, both 2-way and 4-way
> micro-threading mode will be considered.  The default is 6.
> 
> With this, we now have secondary threads which are the primary thread
> for their subcore and therefore need to do the MMU switch.  These
> threads will need to be started even if they have no vcpu to run, so
> we use the vcore pointer in the PACA rather than the vcpu pointer to
> trigger them.
> 
> Signed-off-by: Paul Mackerras <paulus@samba.org>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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