On Tue, 2 Apr 2019 at 20:37, Peter Xu <pet...@redhat.com> wrote: > On Tue, Apr 02, 2019 at 05:06:15PM +0800, Catherine Ho wrote: > > On Tue, 2 Apr 2019 at 15:58, Peter Xu <pet...@redhat.com> wrote: > > > > > On Tue, Apr 02, 2019 at 03:47:16PM +0800, Catherine Ho wrote: > > > > Hi Peter Maydell > > > > > > > > On Tue, 2 Apr 2019 at 11:05, Peter Maydell <peter.mayd...@linaro.org > > > > > wrote: > > > > > > > > > On Tue, 2 Apr 2019 at 09:57, Catherine Ho < > catherine.h...@gmail.com> > > > > > wrote: > > > > > > The root cause is the used idx is moved forward after 1st time > > > incoming, > > > > > and in 2nd time incoming, > > > > > > the last_avail_idx will be incorrectly restored from the saved > device > > > > > state file(not in the ram). > > > > > > > > > > > > I watched this even on x86 for a virtio-scsi disk > > > > > > > > > > > > Any ideas for supporting 2nd time, 3rd time... incoming > restoring? > > > > > > > > > > Does the destination end go through reset between the 1st and 2nd > > > > > > > > > seems not, please see my step below > > > > > > > > > incoming attempts? I'm not a migration expert, but I thought that > > > > > devices were allowed to assume that their state is "state of the > > > > > device following QEMU reset" before the start of an incoming > > > > > migration attempt. > > > > > > > > > > > > > Here is my step: > > > > 1. start guest normal by qemu with shared memory-backend file > > > > 2. stop the vm. save the device state to another file via monitor > migrate > > > > "exec: cat>..." > > > > 3. quit the vm > > > > 4. retore the vm by qemu -incoming "exec:cat ..." > > > > 5. continue the vm via monito, the 1st incoming works fine > > > > 6. quit the vm > > > > 7. retore the vm by qemu -incoming "exec:cat ..." for 2nd time > > > > 8. continue -> error happened > > > > Actually, this can be fixed by forcely restore the idx by > > > > virtio_queue_restore_last_avail_idx(） > > > > But I am sure whether it is reasonable. > > > > > > Yeah I really suspect its validity. > > > > > > IMHO normal migration streams keep the device state and RAM data > > > together in the dumped file, so they always match. > > > > > > In your shared case, the device states are in the dumped file however > > > the RAM data is located somewhere else. After you quit the VM from > > > the 1st incoming migration the RAM is new (because that's a shared > > > memory file) and the device data is still old. They do not match > > > already, then I'd say you can't migrate with that any more. > > > > > > If you want to do that, you'd better take snapshot of the RAM backend > > > file if your filesystem supports (or even simpler, to back it up > > > before hand) before you start any incoming migration. Then with the > > > dumped file (which contains the device states) and that snapshot file > > > (which contains the exact RAM data that matches the device states) > > > you'll alway be able to migrate for as many times as you want. > > > > > > > Understood, thanks Peter Xu > > Is there any feasible way to indicate the snapshot of the RAM backend > file > > is > > matched with the device data? > > >VQ 2 size 0x400 < last_avail_idx 0x1639 - used_idx 0x2688 > > >Failed to load virtio-scsi:virtio > > > > Because I thought reporting above error is not so friendly. Could we add > a > > version id in both RAM backend file and device date file? > > It would be non-trivial I'd say - AFAIK we don't have an existing way > to tag the memory-backend-file content (IIUC that's what you use). > > And since you mentioned about versioning of these states, I just > remembered that even with this you may not be able to get a complete > matched state of the VM, because AFAICT actually besides RAM state & > device state, you probably also need to consider the disk state as > well. After you started the VM of the 1st incoming, there could be > data flushed to the VM backend disk and then that state is changed as > well. So here even if you snapshot the RAM file you'll still lose the > disk state IIUC so it could still be broken. In other words, to make > a migration/snapshot to work you'll need to make all these three > states to match. >
Yes, thanks > > Before we discuss further on the topic... could you share me with your > requirement first? I started to get a bit confused now since when I > thought about shared mem I was thinking about migrating within the > same host to e.g. upgrade the hypervisor but that obviously does not > need you to do incoming migration for multiple times. Then what do > you finally want to achieve? > Actually, I am investigating the ignore-shared capability case support on arm64. This feature is used in Kata containers project as "vm template" The rom reset failure is the first bug. Ok, now I can confirm that doing incoming migration for multiple times is supported. Thanks for the detailed explanation :) B.R. Catherine
