block.c says:
/*
* Yes, BDRV_O_NOCACHE aka O_DIRECT means we have to present a
* write cache to the guest. We do need the fdatasync to flush
* out transactions for block allocations, and we maybe have a
* volatile write cache in our backing device to deal with.
*/
if (flags& (BDRV_O_CACHE_WB|BDRV_O_NOCACHE))
bs->enable_write_cache = 1;
This means that if I start a guest with cache=writethrough and then
restart (or live migrate) it with cache=none, then the guest will see
a change, even though the user only changed the drive's backing, not
something guest visible. In the case of live migration, the guest
will not even notice the change and we may be at risk of data loss.
For 0.13 I propose setting enable_write_cache to true unconditionally.
For 0.12 the question is more difficult, since we'll be changing the
guest ABI. Given that guests are unlikely not to be able to cope with
write caches, and that the alternative is data loss, I believe that's
also the right solution there.