Am 04.11.2013 um 08:24 hat Fam Zheng geschrieben: > > > On Tue 29 Oct 2013 08:12:38 PM CST, Kevin Wolf wrote: > >Am 29.10.2013 um 13:02 hat Paolo Bonzini geschrieben: > >>Il 29/10/2013 12:35, Kevin Wolf ha scritto: > >>>The block layer generally keeps the size of an image cached in > >>>bs->total_sectors so that it doesn't have to perform expensive > >>>operations to get the size whenever it needs it. > >>> > >>>This doesn't work however when using a backend that can change its size > >>>without qemu being aware of it, i.e. passthrough of removable media like > >>>CD-ROMs or floppy disks. For this reason, the caching is disabled when a > >>>removable device is used. > >>> > >>>It is obvious that checking whether the _guest_ device has removable > >>>media isn't the right thing to do when we want to know whether the size > >>>of the host backend can change. To make things worse, non-top-level > >>>BlockDriverStates never have any device attached, which makes qemu > >>>assume they are removable, so drv->bdrv_getlength() is always called on > >>>the protocol layer. In the case of raw-posix, this causes unnecessary > >>>lseek() system calls, which turned out to be rather expensive. > >>> > >>>This patch completely changes the logic and disables bs->total_sectors > >>>caching only for certain block driver types, for which a size change is > >>>expected: host_cdrom and host_floppy; also the raw format in case it > >>>sits on top of one of these protocols, but in the common case the nested > >>>bdrv_getlength() call on the protocol driver will use the cache again > >>>and avoid an expensive drv->bdrv_getlength() call. > >>> > >>>Signed-off-by: Kevin Wolf <kw...@redhat.com> > > > >>raw-win32.c probably needs to have a .has_variable_length=true in > >>bdrv_host_device. Apart from that, > >> > >>Reviewed-by: Paolo Bonzini <pbonz...@redhat.com> > > > >Thanks, good catch. I've added this now. > > This breaks VMDK because it can't read description file: buffer is > empty in bdrv_probe, when the file size is <512 bytes. (for > raw-posix).
Perhaps we should replace bs->total_sectors with bs->total_byts one day... For now, how about introducing a bdrv_getlength_bytes() that always calls the driver and never converts bytes to sectors? Kevin