On Wed, Mar 15, 2023 at 02:17:33AM +0000, 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 live migration postcopy. When many vCPUs fault upon 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 triggering a vCPU exit avoids this contention and can improve > the fault rate by as much as 10x. > --- > Documentation/virt/kvm/api.rst | 37 +++++++++++++++++++++++++++++++--- > include/linux/kvm_host.h | 6 ++++++ > include/uapi/linux/kvm.h | 3 +++ > tools/include/uapi/linux/kvm.h | 2 ++ > virt/kvm/kvm_main.c | 7 ++++++- > 5 files changed, 51 insertions(+), 4 deletions(-) > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index f9ca18bbec879..4932c0f62eb3d 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) call it KVM_MEM_EXIT_ABSENT_MAPPING > > This ioctl allows the user to create, modify or delete a guest physical > memory slot. Bits 0-15 of "slot" specify the slot id and this value > @@ -1342,12 +1343,15 @@ It is recommended that the lower 21 bits of guest_phys_addr and userspace_addr > be identical. This allows large pages in the guest to be backed by large > pages in the host. > > -The flags field supports two flags: KVM_MEM_LOG_DIRTY_PAGES and > -KVM_MEM_READONLY. The former can be set to instruct KVM to keep track of > +The flags field supports three flags > + > +1. KVM_MEM_LOG_DIRTY_PAGES: can be set to instruct KVM to keep track of > writes to memory within the slot. See KVM_GET_DIRTY_LOG ioctl to know how to > -use it. The latter can be set, if KVM_CAP_READONLY_MEM capability allows it, > +use it. > +2. KVM_MEM_READONLY: can be set, if KVM_CAP_READONLY_MEM capability allows it, > to make a new slot read-only. In this case, writes to this memory will be > posted to userspace as KVM_EXIT_MMIO exits. > +3. KVM_MEM_ABSENT_MAPPING_FAULT: see KVM_CAP_MEMORY_FAULT_NOWAIT for details. > > When the KVM_CAP_SYNC_MMU capability is available, changes in the backing of > the memory region are automatically reflected into the guest. For example, an > @@ -7702,10 +7706,37 @@ Through args[0], the capability can be set on a per-exit-reason basis. > Currently, the only exit reasons supported are > > 1. KVM_MEMFAULT_REASON_UNKNOWN (1 << 0) > +2. KVM_MEMFAULT_REASON_ABSENT_MAPPING (1 << 1) > > Memory fault exits with a reason of UNKNOWN should not be depended upon: they > may be added, removed, or reclassified under a stable reason. > > +7.35 KVM_CAP_MEMORY_FAULT_NOWAIT > +-------------------------------- > + > +:Architectures: x86, arm64 > +:Returns: -EINVAL. > + > +The presence of this capability indicates that userspace may pass the > +KVM_MEM_ABSENT_MAPPING_FAULT flag to KVM_SET_USER_MEMORY_REGION to cause KVM_RUN > +to exit to populate 'kvm_run.memory_fault' and exit to userspace (*) in response > +to page faults for which the userspace page tables do not contain present > +mappings. Attempting to enable the capability directly will fail. > + > +The 'gpa' and 'len' fields of kvm_run.memory_fault will be set to the starting > +address and length (in bytes) of the faulting page. 'flags' will be set to > +KVM_MEMFAULT_REASON_ABSENT_MAPPING. > + > +Userspace should determine how best to make the mapping present, then take > +appropriate action. For instance, in the case of absent mappings this might > +involve establishing the mapping for the first time via UFFDIO_COPY/CONTINUE or > +faulting the mapping in using MADV_POPULATE_READ/WRITE. After establishing the > +mapping, userspace can return to KVM to retry the previous memory access. > + > +(*) NOTE: On x86, KVM_CAP_X86_MEMORY_FAULT_EXIT must be enabled for the > +KVM_MEMFAULT_REASON_ABSENT_MAPPING_reason: otherwise userspace will only receive > +a -EFAULT from KVM_RUN without any useful information. I'm not a fan of this architecture-specific dependency. Userspace is already explicitly opting in to this behavior by way of the memslot flag. These sort of exits are entirely orthogonal to the -EFAULT conversion earlier in the series. > 8. Other capabilities. > ====================== > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index d3ccfead73e42..c28330f25526f 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -593,6 +593,12 @@ static inline bool kvm_slot_dirty_track_enabled(const struct kvm_memory_slot *sl > return slot->flags & KVM_MEM_LOG_DIRTY_PAGES; > } > > +static inline bool kvm_slot_fault_on_absent_mapping( > + const struct kvm_memory_slot *slot) Style again... I'd strongly recommend using 'exit' instead of 'fault' in the verbiage of the KVM implementation. I understand we're giving userspace the illusion of a page fault mechanism, but the term is then overloaded in KVM since we handle literal faults from hardware. -- Thanks, Oliver
