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kvm: introduce manual dirty log reprotect

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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 also 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.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2018-10-23 02:36:47 +02:00
parent 8fe65a8299
commit 2a31b9db15
12 changed files with 308 additions and 19 deletions
Download patch file Download diff file Expand all files Collapse all files

5
include/linux/kvm_host.h
View file

@ -449,6 +449,7 @@ struct kvm {
#endif
long tlbs_dirty;
struct list_head devices;
bool manual_dirty_log_protect;
struct dentry *debugfs_dentry;
struct kvm_stat_data **debugfs_stat_data;
struct srcu_struct srcu;
@ -754,6 +755,8 @@ int kvm_get_dirty_log(struct kvm *kvm,
int kvm_get_dirty_log_protect(struct kvm *kvm,
struct kvm_dirty_log *log, bool *flush);
int kvm_clear_dirty_log_protect(struct kvm *kvm,
struct kvm_clear_dirty_log *log, bool *flush);
void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
struct kvm_memory_slot *slot,
@ -762,6 +765,8 @@ void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
struct kvm_dirty_log *log);
int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm,
struct kvm_clear_dirty_log *log);
int kvm_vm_ioctl_irq_line(struct kvm *kvm, struct kvm_irq_level *irq_level,
bool line_status);