Re: [PATCH v3 0/2] KVM: s390: add counters for vsie performance

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Mon, 2023-09-04 at 15:01 +0200, Nico Boehr wrote:
> v3:
> ---
> * rename te -> entry (David)
> * add counters for gmap reuse and gmap create (David)
> 
> v2:
> ---
> * also count shadowing of pages (Janosch)
> * fix naming of counters (Janosch)
> * mention shadowing of multiple levels is counted in each level (Claudio)
> * fix inaccuate commit description regarding gmap notifier (Claudio)
> 
> When running a guest-3 via VSIE, guest-1 needs to shadow the page table
> structures of guest-2.
> 
> To reflect changes of the guest-2 in the _shadowed_ page table structures,
> the _shadowing_ sturctures sometimes need to be rebuilt. Since this is a
> costly operation, it should be avoided whenever possible.
> 
> This series adds kvm stat counters to count the number of shadow gmaps
> created and a tracepoint whenever something is unshadowed. This is a first
> step to try and improve VSIE performance.
> 
> Please note that "KVM: s390: add tracepoint in gmap notifier" has some
> checkpatch --strict findings. I did not fix these since the tracepoint
> definition would then look completely different from all the other
> tracepoints in arch/s390/kvm/trace-s390.h. If you want me to fix that,
> please let me know.
> 
> While developing this, a question regarding the stat counters came up:
> there's usually no locking involved when the stat counters are incremented.
> On s390, GCC accidentally seems to do the right thing(TM) most of the time
> by generating a agsi instruction (which should be atomic given proper
> alignment). However, it's not guaranteed, so would we rather want to go
> with an atomic for the stat counters to avoid losing events? Or do we just
> accept the fact that we might loose events sometimes? Is there anything
> that speaks against having an atomic in kvm_stat?
> 

FWIW the PCI counters (/sys/kernel/debug/pci/<dev>/statistics) use
atomic64_add(). Also, s390's memory model is strong enough that these
are actually just normal loads/stores/adds (see
arch/s390/include/asm/atomic_ops.h) with the generic atomic64_xx()
adding debug instrumentation.