KVM: Add generic support for dirty page logging

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.

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Mario Smarduch <m.smarduch@samsung.com>

2 files changed, 86 insertions, 0 deletions

diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index 3796a2132a06..314950c51d9f 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -40,3 +40,9 @@ config KVM_VFIO
 
 config HAVE_KVM_ARCH_TLB_FLUSH_ALL
        bool
+
+config HAVE_KVM_ARCH_DIRTY_LOG_PROTECT
+       bool
+
+config KVM_GENERIC_DIRTYLOG_READ_PROTECT
+       bool
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d03bd2255801..246cf291c6fd 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -995,6 +995,86 @@ out:
 }
 EXPORT_SYMBOL_GPL(kvm_get_dirty_log);
 
+#ifdef CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT
+/**
+ * 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);
+#endif
+
 bool kvm_largepages_enabled(void)
 {
         return largepages_enabled;