Re: [libvirt] [Qemu-devel] [PATCH v3] Add support for fd: protocol

On Wed, Jul 27, 2011 at 10:11:06AM +0200, Kevin Wolf wrote: > Am 26.07.2011 18:57, schrieb Corey Bryant: >>>>> diff --git a/block/cow.c b/block/cow.c >>>>> index 4cf543c..e17f8e7 100644 >>>>> --- a/block/cow.c >>>>> +++ b/block/cow.c >>>>> @@ -82,6 +82,11 @@ static int cow_open(BlockDriverState *bs, int flags) >>>>> pstrcpy(bs->backing_file, sizeof(bs->backing_file), >>>>> cow_header.backing_file); >>>>> >>>>> + if (bs->backing_file[0] != '\0'&& bdrv_is_fd_protocol(bs)) { >>>>> + /* backing file currently not supported by fd: protocol */ >>>>> + goto fail; >>>>> + } >>> I don't think these checks are strictly needed. Without the check you >>> can open the image itself using an fd, and the backing file using good >>> old raw-posix. We shouldn't decide for our users that this isn't useful. >>> >>> In any case, it would have to move into block.c, you can't modify >>> independent drivers for this. >>> >> >> I understand the point on not modifying independent drivers. >> >> But if the backing file resides on NFS, wouldn't the the proposed >> SELinux changes prevent the open? > > Probably. But what about cases where the backing file is local? Or even > a non-libvirt, non-SELinux use case? > >> Or are you talking about support where libvirt opens the backing file >> and passes the fd to Qemu? If so there was some discussion about future >> support for this here: >> http://lists.gnu.org/archive/html/qemu-devel/2011-06/msg01496.html > > Not really, but implementing this will be a bit easier if you don't > forbid using backing files with fd. In any case, for 'fd:' to be actually used by libvirt, we need to have backing files supported. There are major users of libvirt who rely on NFS and also use backing files, so an 'fd:' impl which can't deal with the backing file problem isn't much use to libvirt.