* Laszlo Ersek (ler...@redhat.com) wrote: > On 02/20/17 12:00, Dr. David Alan Gilbert wrote: > > * Laszlo Ersek (ler...@redhat.com) wrote: > >> On 02/20/17 11:23, Dr. David Alan Gilbert wrote: > >>> * Laszlo Ersek (ler...@redhat.com) wrote: > >>>> CC Dave > >>> > >>> This isn't an area I really understand; but if I'm > >>> reading this right then > >>> vmgenid is stored in fw_cfg? > >>> fw_cfg isn't migrated > >>> > >>> So why should any changes to it get migrated, except if it's already > >>> been read by the guest (and if the guest reads it again aftwards what's > >>> it expected to read?) > >> > >> This is what we have here: > >> - QEMU formats read-only fw_cfg blob with GUID > >> - guest downloads blob, places it in guest RAM > >> - guest tells QEMU the guest-side address of the blob > >> - during migration, guest RAM is transferred > >> - after migration, in the device's post_load callback, QEMU overwrites > >> the GUID in guest RAM with a different value, and injects an SCI > >> > >> I CC'd you for the following reason: Igor reported that he didn't see > >> either the fresh GUID or the SCI in the guest, on the target host, after > >> migration. I figured that perhaps there was an ordering issue between > >> RAM loading and post_load execution on the target host, and so I > >> proposed to delay the RAM overwrite + SCI injection a bit more; > >> following the pattern seen in your commit 90c647db8d59. > >> > >> However, since then, both Ben and myself tested the code with migration > >> (using "virsh save" (Ben) and "virsh managedsave" (myself)), with > >> Windows and Linux guests, and it works for us; there seems to be no > >> ordering issue with the current code (= overwrite RAM + inject SCI in > >> the post_load callback()). > >> > >> For now we don't understand why it doesn't work for Igor (Igor used > >> exec/gzip migration to/from a local file using direct QEMU monitor > >> commands / options, no libvirt). And, copying the pattern seen in your > >> commit 90c647db8d59 didn't help in his case (while it wasn't even > >> necessary for success in Ben's and my testing). > > > > One thing I noticed in Igor's test was that he did a 'stop' on the source > > before the migate, and so it's probably still paused on the destination > > after the migration is loaded, so anything the guest needs to do might > > not have happened until it's started. > > Interesting! I hope Igor can double-check this! > > In the virsh docs, before doing my tests, I read that "managedsave" > optionally took --running or --paused: > > Normally, starting a managed save will decide between running or > paused based on the state the domain was in when the save was done; > passing either the --running or --paused flag will allow overriding > which state the start should use. > > I didn't pass any such flag ultimately, and I didn't stop the guests > before the managedsave. Indeed they continued execution right after > being loaded with "virsh start". > > (Side point: managedsave is awesome. :) )
If I've followed the bread crumbs correctly, I think managedsave is just using a migrate to fd anyway, so the same code. > > > > You say; > > 'guest tells QEMU the guest-side address of the blob' > > how is that stored/migrated/etc ? > > It is a uint8_t[8] array (little endian representation), linked into > another (writeable) fw_cfg entry, and it's migrated explicitly (it has a > descriptor in the device's vmstate descriptor). The post_load callback > relies on this array being restored before the migration infrastructure > calls post_load. RAM normally comes back before other devices, so you should be OK; although we frequently have problems with devices reading from RAM during device init before migration has started, or writing to it after migration has finished on the source. Dave > > Thanks > Laszlo -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
