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. :) ) > > 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. Thanks Laszlo