[PATCH v13 2/7] KVM: Add generic support for dirty page logging

Mario Smarduch
2014-11-07T00:40:43Z

kvm_get_dirty_log() provides generic handling of dirty bitmap, currently reused
by several architectures. Building on that we intrdoduce
kvm_get_dirty_log_protect() adding write protection to mark these pages dirty
for future write access, before next KVM_GET_DIRTY_LOG ioctl call from user
space.

Signed-off-by: Mario Smarduch <m.smarduch@samsung.com>
---
include/linux/kvm_host.h | 9 +++++
virt/kvm/kvm_main.c | 95 ++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 104 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index a4c33b3..c55dd75 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -590,6 +590,15 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext);

int kvm_get_dirty_log(struct kvm *kvm,
struct kvm_dirty_log *log, int *is_dirty);
+
+int kvm_get_dirty_log_protect(struct kvm *kvm,
+ struct kvm_dirty_log *log, bool *is_dirty);
+
+void kvm_arch_mmu_write_protect_pt_masked(struct kvm *kvm,
+ struct kvm_memory_slot *slot,
+ gfn_t gfn_offset,
+ unsigned long mask);
+
int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
struct kvm_dirty_log *log);

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 887df87..f017760 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -981,6 +981,101 @@ out:
}
EXPORT_SYMBOL_GPL(kvm_get_dirty_log);

+#if defined(CONFIG_S390) || defined(CONFIG_PPC) || defined(CONFIG_MIPS) || \
+ defined(CONFIG_IA64) || defined(CONFIG_X86) || defined(CONFIG_ARM) || \
+ defined(CONFIG_ARM64)
+/*
+ * For architectures that don't use kvm_get_dirty_log_protect() for dirty page
+ * logging, calling this function is illegal. Otherwise the function is defined
+ * in arch subtree and this restriction is removed.
+ */
+void kvm_arch_mmu_write_protect_pt_masked(struct kvm *kvm,
+ struct kvm_memory_slot *slot,
+ gfn_t gfn_offset,
+ unsigned long mask)
+{
+ BUG();
+}
+#endif
+
+/**
+ * kvm_get_dirty_log_protect - get a snapshot of dirty pages, and if any pages
+ * are dirty write protect them for next write.
+ * @kvm: pointer to kvm instance
+ * @log: slot id and address to which we copy the log
+ * @is_dirty: flag set if any page is dirty
+ *
+ * We need to keep it in mind that VCPU threads can write to the bitmap
+ * concurrently. So, to avoid losing track of dirty pages we keep the
+ * following order:
+ *
+ * 1. Take a snapshot of the bit and clear it if needed.
+ * 2. Write protect the corresponding page.
+ * 3. Copy the snapshot to the userspace.
+ * 4. Upon return caller flushes TLB's if needed.
+ *
+ * Between 2 and 4, the guest may write to the page using the remaining TLB
+ * entry. This is not a problem because the page is reported dirty using
+ * the snapshot taken before and step 4 ensures that writes done after
+ * exiting to userspace will be logged for the next call.
+ *
+ */
+int kvm_get_dirty_log_protect(struct kvm *kvm,
+ struct kvm_dirty_log *log, bool *is_dirty)
+{
+ struct kvm_memory_slot *memslot;
+ int r, i;
+ unsigned long n;
+ unsigned long *dirty_bitmap;
+ unsigned long *dirty_bitmap_buffer;
+
+ r = -EINVAL;
+ if (log->slot >= KVM_USER_MEM_SLOTS)
+ goto out;
+
+ memslot = id_to_memslot(kvm->memslots, log->slot);
+
+ dirty_bitmap = memslot->dirty_bitmap;
+ r = -ENOENT;
+ if (!dirty_bitmap)
+ goto out;
+
+ n = kvm_dirty_bitmap_bytes(memslot);
+
+ dirty_bitmap_buffer = dirty_bitmap + n / sizeof(long);
+ memset(dirty_bitmap_buffer, 0, n);
+
+ spin_lock(&kvm->mmu_lock);
+ *is_dirty = false;
+ for (i = 0; i < n / sizeof(long); i++) {
+ unsigned long mask;
+ gfn_t offset;
+
+ if (!dirty_bitmap[i])
+ continue;
+
+ *is_dirty = true;
+
+ mask = xchg(&dirty_bitmap[i], 0);
+ dirty_bitmap_buffer[i] = mask;
+
+ offset = i * BITS_PER_LONG;
+ kvm_arch_mmu_write_protect_pt_masked(kvm, memslot, offset,
+ mask);
+ }
+
+ spin_unlock(&kvm->mmu_lock);
+
+ r = -EFAULT;
+ if (copy_to_user(log->dirty_bitmap, dirty_bitmap_buffer, n))
+ goto out;
+
+ r = 0;
+out:
+ return r;
+}
+EXPORT_SYMBOL_GPL(kvm_get_dirty_log_protect);
+
bool kvm_largepages_enabled(void)
{
return largepages_enabled;
--
1.7.9.5