On Thu, May 11, 2023 at 04:59:13PM -0700, Sean Christopherson wrote: > Convert all "runtime" assertions, i.e. assertions that can be triggered > while running vCPUs, from WARN_ON() to WARN_ON_ONCE(). Every WARN in the > MMU that is tied to running vCPUs, i.e. not contained to loading and > initializing KVM, is likely to fire _a lot_ when it does trigger. E.g. if > KVM ends up with a bug that causes a root to be invalidated before the > page fault handler is invoked, pretty much _every_ page fault VM-Exit > triggers the WARN. > > If a WARN is triggered frequently, the resulting spam usually causes a lot > of damage of its own, e.g. consumes resources to log the WARN and pollutes > the kernel log, often to the point where other useful information can be > lost. In many case, the damage caused by the spam is actually worse than > the bug itself, e.g. KVM can almost always recover from an unexpectedly > invalid root. > > On the flip side, warning every time is rarely helpful for debug and > triage, i.e. a single splat is usually sufficient to point a debugger in > the right direction, and automated testing, e.g. syzkaller, typically runs > with warn_on_panic=1, i.e. will never get past the first WARN anyways. On the topic of syzkaller, we should get them to test with CONFIG_KVM_PROVE_MMU once it's available. > > Lastly, when an assertions fails multiple times, the stack traces in KVM > are almost always identical, i.e. the full splat only needs to be captured > once. And _if_ there is value in captruing information about the failed > assert, a ratelimited printk() is sufficient and less likely to rack up a > large amount of collateral damage. These are all good arguments and I think they apply to KVM_MMU_WARN_ON() as well. Should we convert that to _ONCE() too?
