On 2024-03-08 02:07 PM, Sean Christopherson wrote: > On Thu, Feb 15, 2024, Anish Moorthy wrote: > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > > index 9f5d45c49e36..bf7bc21d56ac 100644 > > --- a/Documentation/virt/kvm/api.rst > > +++ b/Documentation/virt/kvm/api.rst > > @@ -1353,6 +1353,7 @@ yet and must be cleared on entry. > > #define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0) > > #define KVM_MEM_READONLY (1UL << 1) > > #define KVM_MEM_GUEST_MEMFD (1UL << 2) > > + #define KVM_MEM_EXIT_ON_MISSING (1UL << 3) > > David M., > > Before this gets queued anywhere, a few questions related to the generic KVM > userfault stuff you're working on: > > 1. Do you anticipate reusing KVM_MEM_EXIT_ON_MISSING to communicate that a vCPU > should exit to userspace, even for guest_memfd? Or are you envisioning the > "data invalid" gfn attribute as being a superset? > > We danced very close to this topic in the PUCK call, but I don't _think_ we > ever explicitly talked about whether or not KVM_MEM_EXIT_ON_MISSING would > effectively be obsoleted by a KVM_SET_MEMORY_ATTRIBUTES-based "invalid data" > flag. > > I was originally thinking that KVM_MEM_EXIT_ON_MISSING would be re-used, > but after re-watching parts of the PUCK recording, e.g. about decoupling > KVM from userspace page tables, I suspect past me was wrong. No I don't anticipate reusing KVM_MEM_EXIT_ON_MISSING. The plan is to introduce a new gfn attribute and exit to userspace based on that. I do forsee having an on/off switch for the new attribute, but it wouldn't make sense to reuse KVM_MEM_EXIT_ON_MISSING for that. > > 2. What is your best guess as to when KVM userfault patches will be available, > even if only in RFC form? We're aiming for the end of April for RFC with KVM/ARM support. > > The reason I ask is because Oliver pointed out (off-list) that (a) Google is the > primary user for KVM_MEM_EXIT_ON_MISSING, possibly the _only_ user for the > forseeable future, and (b) if Google moves on to KVM userfault before ever > ingesting KVM_MEM_EXIT_ON_MISSING from upstream, then we'll have effectively > added dead code to KVM's eternal ABI.
