[Qemu-devel] bs->enable_write_cache and the guest ABI

[Avi Kivity <avi@redhat.com>]

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.

-- 
error compiling committee.c: too many arguments to function