[libvirt] [PATCH] qemu: Refresh state before starting the VCPUs

[Peter Krempa]

On Mon, Feb 04, 2019 at 14:41:05 +0100, Marc Hartmayer wrote:
> On Mon, Feb 04, 2019 at 02:37 PM +0100, Marc Hartmayer <mhartmay at linux.ibm.com> wrote:
> > On Mon, Feb 04, 2019 at 01:41 PM +0100, Peter Krempa <pkrempa at redhat.com> wrote:
> >> On Mon, Feb 04, 2019 at 13:36:24 +0100, Marc Hartmayer wrote:
> >>> For normal starts (no incoming migration) the refresh of the QEMU
> >>> state must be done before the VCPUs getting started since otherwise
> >>> there might be a race condition between a possible shutdown of the
> >>> guest OS and the QEMU monitor queries.
> >>>
> >>> This fixes "qemu: migration: Refresh device information after
> >>> transferring state" (93db7eea1b864).
> >>>
> >>> Signed-off-by: Marc Hartmayer <mhartmay at linux.ibm.com>
> >>> ---
> >>>  src/qemu/qemu_process.c | 20 +++++++++++---------
> >>>  1 file changed, 11 insertions(+), 9 deletions(-)
> >>>
> >>> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> >>> index dace5aaca102..2a3763f40d49 100644
> >>> --- a/src/qemu/qemu_process.c
> >>> +++ b/src/qemu/qemu_process.c
> >>> @@ -6929,10 +6929,17 @@ qemuProcessStart(virConnectPtr conn,
> >>>      }
> >>>      relabel = true;
> >>>
> >>> -    if (incoming &&
> >>> -        incoming->deferredURI &&
> >>> -        qemuMigrationDstRun(driver, vm, incoming->deferredURI, asyncJob) < 0)
> >>> -        goto stop;
> >>> +    if (incoming) {
> >>> +        if (incoming->deferredURI &&
> >>> +            qemuMigrationDstRun(driver, vm, incoming->deferredURI, asyncJob) < 0)
> >>> +            goto stop;
> >>> +    } else {
> >>
> >> This logic does not seem right ...
> >
> > I’m not familiar with the usage of this function for migration… so
> > there's a good chance I'm missing something.
> >
> >>
> >>> +        /* Refresh state of devices from QEMU. During migration this
> >>> +         * needs to happen after the state information is fully
> >>> +         * transferred. */
> >>
> >> as this comment clearly states that this should happen after
> >> migration.
> >
> > Is qemuProcessFinishStartup
>      ^^^^^^^^^^^^^^^^^^^^^^^^
>      qemuProcessRefreshState
> 
> > not called explicitly in qemuMigrationFinish
> > for migration?
> >

Yes it is, in this case the comment is wrong/misleading so it should be
fixed.
> >>
> >> Here it would happen only when migration is not done.
> >
> > Without this patch qemuProcessRefreshState is called after
> > qemuProcessFinishStartup if (!incoming). Now it’s called directly before
> > qemuProcessFinishStartup if (!incoming). Why should this be wrong now?
> > What happens in qemuProcessFinishStartup?
> >
> > Before your change in 93db7eea1b86408e the function call
> > 'qemuProcessRefreshState' was done in qemuProcessFinishStartup (without
> > any condition).

As I've said in the other thread I think this fix is okay if we fix the
comment. I'll do this in a moment so you don't need to send another
patch.
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