[libvirt] QEMU's File Descriptors

[Christopher Pereira]

Dear Martin,

Does libvirt currently reopen image files when resuming a VM?

There is an issue of an IO Error after restarting a gluster volume and probably doing other maintenance tasks (VM will not resume) which probably invalidates QEMU's File Descriptors.

It's an old BZ.

Martin Kletzander <mkletzan at redhat.com> wrote:

>On Tue, Feb 24, 2015 at 09:24:51AM -0500, Stefan Berger wrote:
>>On 02/24/2015 08:34 AM, Martin Kletzander wrote:
>>>On Mon, Feb 23, 2015 at 06:50:46AM -0500, Stefan Berger wrote:
>>>>Implement virCommandPassFDGetFDIndex to determine the index a given
>>>>file descriptor will have when passed to the child process. When this
>>>>function is called, a flag is set to prevent the reordering of the
>>>>file descriptors.
>>>>
>>>>Signed-off-by: Stefan Berger <stefanb at linux.vnet.ibm.com>
>>>>---
>>>>src/libvirt_private.syms |  1 +
>>>>src/util/vircommand.c    | 33 +++++++++++++++++++++++++++++++++
>>>>src/util/vircommand.h    |  3 +++
>>>>3 files changed, 37 insertions(+)
>>>>
>>>>diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
>>>>index c156b40..aadb833 100644
>>>>--- a/src/libvirt_private.syms
>>>>+++ b/src/libvirt_private.syms
>>>>@@ -1213,6 +1213,7 @@ virCommandNewArgList;
>>>>virCommandNewArgs;
>>>>virCommandNonblockingFDs;
>>>>virCommandPassFD;
>>>>+virCommandPassFDGetFDIndex;
>>>>virCommandPassListenFDs;
>>>>virCommandRawStatus;
>>>>virCommandRequireHandshake;
>>>>diff --git a/src/util/vircommand.c b/src/util/vircommand.c
>>>>index 6527d85..2616446 100644
>>>>--- a/src/util/vircommand.c
>>>>+++ b/src/util/vircommand.c
>>>>@@ -67,6 +67,7 @@ enum {
>>>>    VIR_EXEC_RUN_SYNC   = (1 << 3),
>>>>    VIR_EXEC_ASYNC_IO   = (1 << 4),
>>>>    VIR_EXEC_LISTEN_FDS = (1 << 5),
>>>>+    VIR_EXEC_FIXED_FDS  = (1 << 6),
>>>
>>>The descriptors shouldn't get reordered *unless* there is
>>>VIR_EXEC_LISTEN_FDS flag added, so FIXED_FDS shouldn't be necessary.
>>>Or is there a bug in that approach that needs fixing?
>>
>>Hm, it depends on the order in which APIs are called where we either
>>need to protect against re-ordering (if we use indices) or we cannot
>>allow indices to be used if we need to reorder them. So, actually the
>>way the patch is, it isn't correct, either.
>>
>>The problem of having to prevent the reordering of file descriptors
>>when we are building the QEMU command line with file descriptors
>>already exists today. So maybe I could just remove this flag and the
>>related check knowing that reordering of file descriptors and building
>>the QEMU command line that contains file descriptor (from before the
>>order) would not be allowed today without any of the patches I am
>>trying to add.
>>
>
>You surely and safely can.  The point behind reordering FDs was that
>when you're using FD passing as done by systemd, those FDs passed must
>start from number 3.  However, because we are already passing a bunch
>of file descriptors to QEMU and other commands, we *must not* reorder
>any file descriptors because it wouldn't work, of course.
>
>Martin
>
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