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Re: [libvirt] [Qemu-devel] [PATCH v4 0/7] file descriptor passing using pass-fd



On Mon, 09 Jul 2012 13:35:19 -0400
Corey Bryant <coreyb linux vnet ibm com> wrote:

> 
> 
> On 07/09/2012 11:46 AM, Kevin Wolf wrote:
> > Am 09.07.2012 17:05, schrieb Corey Bryant:
> >> I'm not sure this is an issue with current design.  I know things have
> >> changed a bit as the email threads evolved, so I'll paste the current
> >> design that I am working from.  Please let me know if you still see any
> >> issues.
> >>
> >> FD passing:
> >> -----------
> >> New monitor commands enable adding/removing an fd to/from a set.  New
> >> monitor command query-fdsets enables querying of current monitor fdsets.
> >>    The set of fds should all refer to the same file, with each fd having
> >> different access flags (ie. O_RDWR, O_RDONLY).  qemu_open can then dup
> >> the fd that has the matching access mode flags.
> >>
> >> Design points:
> >> --------------
> >> 1. add-fd
> >> -> fd is passed via SCM rights and qemu adds fd to first unused fdset
> >> (e.g. /dev/fdset/1)
> >> -> add-fd monitor function initializes the monitor inuse flag for the
> >> fdset to true
> >> -> add-fd monitor function initializes the remove flag for the fd to false
> >> -> add-fd returns fdset number and received fd number (e.g fd=3) to caller
> >>
> >> 2. drive_add file=/dev/fdset/1
> >> -> qemu_open uses the first fd in fdset1 that has access flags matching
> >> the qemu_open action flags and has remove flag set to false
> >> -> qemu_open increments refcount for the fdset
> >> -> Need to make sure that if a command like 'device-add' fails that
> >> refcount is not incremented
> >>
> >> 3. add-fd fdset=1
> >> -> fd is passed via SCM rights
> >> -> add-fd monitor function adds the received fd to the specified fdset
> >> (or fails if fdset doesn't exist)
> >> -> add-fd monitor function initializes the remove flag for the fd to false
> >> -> add-fd returns fdset number and received fd number (e.g fd=4) to caller
> >>
> >> 4. block-commit
> >> -> qemu_open performs "reopen" by using the first fd from the fdset that
> >> has access flags matching the qemu_open action flags and has remove flag
> >> set to false
> >> -> qemu_open increments refcount for the fdset
> >> -> Need to make sure that if a command like 'block-commit' fails that
> >> refcount is not incremented
> >>
> >> 5. remove-fd fdset=1 fd=4
> >> -> remove-fd monitor function fails if fdset doesn't exist
> >> -> remove-fd monitor function turns on remove flag for fd=4
> >
> > What was again the reason why we keep removed fds in the fdset at all?
> 
> Because if refcount is > 0 for the fd set, then the fd could be in use 
> by a block device.  So we keep it around until refcount is decremented 
> to zero, at which point it is safe to close.

But then the refcount is associated with the set, not with any particular fd.


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