[asdbffg]

VM certainly does "call underlying host kernel operation", it just does so indirectly — the guest userspace calls fadvise(), kernel implementation of fadvise() asks the virtio disk driver to perform particular read/writes, the virtio driver asks underlying kernel disk driver to read/write individual disk sectors (without knowing, that they are related to specific file in guest filesystem).

This specific bug was caused by putting high load on "kernel dentry cache", e.g. a contention for memory structure, present in kernel memory. Guests normally don't share memory, so contending for it was avoided.

Incidentally, there are situations, when different guests can compete for same memory — when VM uses so-called "memory deduplication" techniques. Which is why enabling that stuff on production systems may be a bad idea.

[cjhanks]

Thanks for explanation. I've seen the damage of KSMD as well.