Better late than never right? :) On Mon, Apr 24, 2023 at 02:02:49PM -0700, Sean Christopherson wrote: > On Wed, Apr 12, 2023, Anish Moorthy wrote: > > Add documentation, memslot flags, useful helper functions, and the > > actual new capability itself. > > > > Memory fault exits on absent mappings are particularly useful for > > userfaultfd-based postcopy live migration. When many vCPUs fault on a > > single userfaultfd the faults can take a while to surface to userspace > > due to having to contend for uffd wait queue locks. Bypassing the uffd > > entirely by returning information directly to the vCPU exit avoids this > > contention and improves the fault rate. > > > > Suggested-by: James Houghton <jthoughton@xxxxxxxxxx> > > Signed-off-by: Anish Moorthy <amoorthy@xxxxxxxxxx> > > --- > > Documentation/virt/kvm/api.rst | 31 ++++++++++++++++++++++++++++--- > > include/linux/kvm_host.h | 7 +++++++ > > include/uapi/linux/kvm.h | 2 ++ > > tools/include/uapi/linux/kvm.h | 1 + > > virt/kvm/kvm_main.c | 3 +++ > > 5 files changed, 41 insertions(+), 3 deletions(-) > > > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > > index f174f43c38d45..7967b9909e28b 100644 > > --- a/Documentation/virt/kvm/api.rst > > +++ b/Documentation/virt/kvm/api.rst > > @@ -1312,6 +1312,7 @@ yet and must be cleared on entry. > > /* for kvm_userspace_memory_region::flags */ > > #define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0) > > #define KVM_MEM_READONLY (1UL << 1) > > + #define KVM_MEM_ABSENT_MAPPING_FAULT (1UL << 2) > > This name is both too specific and too vague. It's too specific because it affects > more than just "absent" mappings, it will affect any page fault that can't be > resolved by fast GUP, i.e. I'm objecting for all the same reasons I objected to > the exit reason being name KVM_MEMFAULT_REASON_ABSENT_MAPPING. It's too vague > because it doesn't describe what behavior the flag actually enables in any way. > > I liked the "nowait" verbiage from the RFC. "fast_only" is an ok alternative, > but that's much more of a kernel-internal name. > > Oliver, you had concerns with using "fault" in the name, is something like > KVM_MEM_NOWAIT_ON_PAGE_FAULT or KVM_MEM_NOWAIT_ON_FAULT palatable? IMO, "fault" > is perfectly ok, we just need to ensure it's unlikely to be ambiguous for userspace. Yeah, I can get over it. Slight preference towards KVM_MEM_NOWAIT_ON_FAULT, fewer characters and still gets the point across. -- Thanks, Oliver
