On Thu, Jan 19, 2017 at 10:19:29AM -0600, Eric Blake wrote: > On 01/19/2017 09:49 AM, Daniel P. Berrange wrote: > > On Thu, Jan 19, 2017 at 10:38:14PM +0800, Fam Zheng wrote: > >> This implements open flag sensible image locking for local file > >> and host device protocol. > >> > >> virtlockd in libvirt locks the first byte, so we start looking at the > >> file bytes from 1. > >> > >> Quoting what was proposed by Kevin Wolf <kw...@redhat.com>, there are > >> four locking modes by combining two bits (BDRV_O_RDWR and > >> BDRV_O_SHARE_RW), and implemented by taking two locks: > >> > > >> +/* Posix file locking bytes. Libvirt takes byte 0, so start from byte 1. > >> */ > >> +#define RAW_LOCK_BYTE_MIN 1 > >> +#define RAW_LOCK_BYTE_NO_OTHER_WRITER 1 > >> +#define RAW_LOCK_BYTE_WRITE 2 > > > > ...would you mind if QEMU started from say byte 10, leaving the first 10 > > reserved for libvirt uses. This lets libvirt have a continuous space for > > its own usage if we want to use more bytes > > Thankfully, fcntl() locks can be taken beyond end-of-file, so I think > we're okay making an arbitrarily larger range of bytes reserved for each > process, even in the unlikely corner case of passing files smaller than > 512 bytes.
Not so unlikely. libguestfs actually detects and works around this case because qemu was (possibly still is) very broken when you pass small files. https://github.com/libguestfs/libguestfs/blob/master/src/drives.c#L395-L441 Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org