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.
