On Wed, Mar 06, 2024, Kai Huang wrote: > On 5/03/2024 4:51 am, Sean Christopherson wrote: > > In other words, KVM_EXIT_MEMORY_FAULT essentially communicates to userspace that > > (a) userspace can likely fix whatever badness triggered the -EFAULT, and (b) that > > KVM is in a state where fixing the underlying problem and resuming the guest is > > safe, e.g. won't corrupt the guest (because KVM is in a half-baked state). > > > > Sure. One small issue might be that, in a later code check, we actually > return KVM_EXIT_MEMORY_FAULT when private fault hits RET_PF_EMULATION -- see > your patch: > > [PATCH 01/16] KVM: x86/mmu: Exit to userspace with -EFAULT if private fault > hits emulation > > So here if we just return -EFAULT w/o reporting KVM_EXIT_MEMORY_FAULT when > private+reserved is hit, it seems there's a little bit inconsistency here. It's intentionally inconsistent. -EFAULT without KVM_EXIT_MEMORY_FAULT is essentially KVM saying "something bad happened, and it can't be fixed", whereas exiting with KVM_EXIT_MEMORY_FAULT says "there's an issue, but you may be able to resolve it". The ABI is a bit messy, e.g. in some ways it would be cleaner if KVM returned '0'. But doing that in a backwards compatible way would have required a rather ugly opt-in, and it would also make it more tedious to extend KVM_EXIT_MEMORY_FAULT, e.g. pairing it with -EHWPOISON didn't require any new flags.
