On Wed, Jul 17, 2024 at 8:34 AM Wang, Wei W <wei.w.wang@xxxxxxxxx> wrote: > > On Thursday, July 11, 2024 7:42 AM, James Houghton wrote: > > The first prong for enabling KVM Userfault support for x86 is to be able to > > inform userspace of userfaults. We know when userfaults occurs when > > fault->pfn comes back as KVM_PFN_ERR_FAULT, so in > > kvm_mmu_prepare_memory_fault_exit(), simply check if fault->pfn is indeed > > KVM_PFN_ERR_FAULT. This means always setting fault->pfn to a known value (I > > have chosen KVM_PFN_ERR_FAULT) before calling > > kvm_mmu_prepare_memory_fault_exit(). > > > > The next prong is to unmap pages that are newly userfault-enabled. Do this in > > kvm_arch_pre_set_memory_attributes(). > > Why is there a need to unmap it? > I think a userfault is triggered on a page during postcopy when its data has not yet > been fetched from the source, that is, the page is never accessed by guest on the > destination and the page table leaf entry is empty. > You're right that it's not strictly necessary for implementing post-copy. This just comes down to the UAPI we want: does ATTRIBUTE_USERFAULT mean "KVM will be unable to access this memory; any attempt to access it will generate a userfault" or does it mean "accesses to never-accessed, non-prefaulted memory will generate a userfault." I think the former (i.e., the one implemented in this RFC) is slightly clearer and slightly more useful. Userfaultfd does the latter: 1. MAP_PRIVATE|MAP_ANONYMOUS + UFFDIO_REGISTER_MODE_MISSING: if nothing is mapped (i.e., major page fault) 2. non-anonymous VMA + UFFDIO_REGISTER_MODE_MISSING: if the page cache does not contain a page 3. MAP_SHARED + UFFDIO_REGISTER_MODE_MINOR: if the page cache *contains* a page, but we got a fault anyway But in all of these cases, we have a way to start getting userfaults for already-accessed memory: for (1) and (3), MADV_DONTNEED, and for (2), fallocate(FALLOC_FL_PUNCH_HOLE). Even if we didn't have MADV_DONTNEED (as used to be the case with HugeTLB), we can use PROT_NONE to prevent anyone from mapping anything in between an mmap() and a UFFDIO_REGISTER. This has been useful for me. With KVM, we have neither of these tools (unless we include them here), AFAIA.
