Messages in this thread | | | From | Paolo Bonzini <> | Subject | [PATCH 0/3] kvm: split retrieval and clearing of dirty log | Date | Mon, 26 Nov 2018 17:54:10 +0100 |
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There are two problems with KVM_GET_DIRTY_LOG. First, and less important, it can take kvm->mmu_lock for an extended period of time. Second, its user can actually see many false positives in some cases. The latter is due to a benign race like this:
1. KVM_GET_DIRTY_LOG returns a set of dirty pages and write protects them. 2. The guest modifies the pages, causing them to be marked ditry. 3. Userspace actually copies the pages. 4. KVM_GET_DIRTY_LOG returns those pages as dirty again, even though they were not written to since (3).
This is especially a problem for large guests, where the time between (1) and (3) can be substantial. This patch introduces a new capability which, when enabled, makes KVM_GET_DIRTY_LOG not write-protect the pages it returns. Instead, userspace has to explicitly clear the dirty log bits just before using the content of the page. The new KVM_CLEAR_DIRTY_LOG ioctl can operate on a 64-page granularity rather than requiring to sync a full memslot. This way the mmu_lock is taken for small amounts of time, and only a small amount of time will pass between write protection of pages and the sending of their content.
This is entirely implemented in generic code, but only users of kvm_get_dirty_log_protect get the support (that is x86_64 and ARM).
Paolo Bonzini (3): kvm: make KVM_CAP_ENABLE_CAP_VM architecture agnostic kvm: rename last argument to kvm_get_dirty_log_protect kvm: introduce manual dirty log reprotect
Documentation/virtual/kvm/api.txt | 78 ++++++++++- arch/mips/kvm/mips.c | 29 +++- arch/powerpc/kvm/powerpc.c | 14 +- arch/s390/kvm/kvm-s390.c | 11 +- arch/x86/kvm/x86.c | 47 ++++--- include/linux/kvm_host.h | 9 +- include/uapi/linux/kvm.h | 15 +++ tools/testing/selftests/kvm/Makefile | 2 + tools/testing/selftests/kvm/clear_dirty_log_test.c | 2 + tools/testing/selftests/kvm/dirty_log_test.c | 19 +++ tools/testing/selftests/kvm/include/kvm_util.h | 2 + tools/testing/selftests/kvm/lib/kvm_util.c | 13 ++ virt/kvm/arm/arm.c | 22 ++- virt/kvm/kvm_main.c | 147 ++++++++++++++++++--- 14 files changed, 345 insertions(+), 65 deletions(-) create mode 100644 tools/testing/selftests/kvm/clear_dirty_log_test.c
-- 1.8.3.1
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