From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:35394) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SoI4T-0002Nd-Rv for qemu-devel@nongnu.org; Mon, 09 Jul 2012 13:48:31 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1SoI4R-0004fI-Gg for qemu-devel@nongnu.org; Mon, 09 Jul 2012 13:48:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:4393) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SoI4R-0004f1-7O for qemu-devel@nongnu.org; Mon, 09 Jul 2012 13:48:27 -0400 Date: Mon, 9 Jul 2012 14:48:42 -0300 From: Luiz Capitulino Message-ID: <20120709144842.123febe2@doriath.home> In-Reply-To: <4FFB1657.1090405@linux.vnet.ibm.com> References: <1340390174-7493-1-git-send-email-coreyb@linux.vnet.ibm.com> <20120626091004.GA14451@redhat.com> <4FE9A0F0.2050809@redhat.com> <20120626175045.2c7011b3@doriath.home> <4FEA37A9.10707@linux.vnet.ibm.com> <4FEA3D9C.8080205@redhat.com> <4FF21A67.8010100@linux.vnet.ibm.com> <4FF31265.1000308@linux.vnet.ibm.com> <4FF316C9.5020100@redhat.com> <4FF31CFD.7030508@linux.vnet.ibm.com> <4FF325C8.4060401@redhat.com> <4FF33004.5030909@linux.vnet.ibm.com> <4FF33349.10404@redhat.com> <4FF3381D.40101@linux.vnet.ibm.com> <4FF3FA22.6090400@redhat.com> <4FF5AD90.8000305@linux.vnet.ibm.com> <20120709110510.12214347@doriath.home> <4FFAF334.9000807@linux.vnet.ibm.com> <4FFAFCB8.8020508@redhat.com> <4FFB1657.1090405@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v4 0/7] file descriptor passing using pass-fd List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Corey Bryant Cc: Kevin Wolf , aliguori@us.ibm.com, stefanha@linux.vnet.ibm.com, libvir-list@redhat.com, qemu-devel@nongnu.org, pbonzini@redhat.com, Eric Blake On Mon, 09 Jul 2012 13:35:19 -0400 Corey Bryant 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.