On Tue, May 15, 2018 at 03:50:54PM +0200, Maxime Coquelin wrote: > Hi Tiwei, > > I just see I missed to reply to your comment on my commit message: > > On 05/03/2018 01:56 PM, Tiwei Bie wrote: > > On Mon, Apr 30, 2018 at 05:59:54PM +0200, Maxime Coquelin wrote: > > > This patch caches all dirty pages logging until the used ring index > > > is updated. These dirty pages won't be accessed by the guest as > > > long as the host doesn't give them back to it by updating the > > > index. > > Below sentence in above commit message isn't the reason why > > we can cache the dirty page logging. Right? > > > > """ > > These dirty pages won't be accessed by the guest as > > long as the host doesn't give them back to it by updating the > > index. > > That's my understanding. > As long as the used index is not updated, the guest will not process > the descs. > If the migration converges between the time the descs are written, > and the time the used index is updated on source side. Then the guest > running on destination will not see the descriptors as used but as > available, and so will be overwritten by the vhost backend on > destination.
If my understanding is correct, theoretically the vhost backend can cache all the dirty page loggings before it responds to the GET_VRING_BASE messages. Below are the steps how QEMU live migration works (w/o postcopy): 1. Syncing dirty pages between src and dst; 2. The dirty page sync converges; 3. The src QEMU sends GET_VRING_BASE to vhost backend; 4. Vhost backend still has a chance to log some dirty pages before responding the GET_VRING_BASE messages; 5. The src QEMU receives GET_VRING_BASE response (which means the device has stopped); 6. QEMU sync the remaining dirty pages; 7. QEMU on the dst starts running. (The steps 3~6 are the downtime which we want to minimize) So I think the words in commit log isn't really related to why we can cache the dirty page loggings. Best regards, Tiwei Bie > > Regards, > Maxime