On Tue, Mar 28, 2023 at 04:36:29PM +0000, Durrant, Paul wrote: > > -----Original Message----- > > From: Stefan Hajnoczi <stefa...@redhat.com> > > Sent: 28 March 2023 16:51 > > To: Woodhouse, David <d...@amazon.co.uk>; Durrant, Paul > > <pdurr...@amazon.co.uk> > > Cc: qemu-devel@nongnu.org; qemu-bl...@nongnu.org > > Subject: [EXTERNAL] aio_set_event_notifier(is_external=true) in Xen code? > > > > Hi, > > I'm removing the aio_disable_external() API from QEMU and noticed that > > Xen code calls aio_set_fd_handler(is_external=true) in hw/xen/xen-bus.c > > and hw/i386/kvm/xen_xenstore.c. > > > > It wasn't clear to me whether is_external=true is necessary here. > > is_external=true is mainly used to temporarily pause I/O submission in > > the QEMU block layer. Maybe is_external=true was chosen out of caution > > but actually has no effect in this code. > > > > Does the Xen code rely on is_external=true? > > > That's a good question. The call in xen-bus.c has been there since commit > 83361a8a1f932, which was when it substituted the old call to > qemu_set_fd_handler(). I suspect this was out of caution (or possibly > misunderstanding) at the time, although setting the call to > xen_device_set_event_channel_context() in xen_block_dataplane_stop() does > suggest it may be happening while I/O could be in progress so it could have > been in response to problems caught in testing. > I suspect the code in xen_xenstore.c just copied what xen-bus.c did. > > Sorry I can't give you a definitive answer... it's all rather a long time ago.
Thanks, that helped! I'll study hw/block/dataplane/xen-block.c and make sure it has a safe alternative to is_external=true. I think is_external=false is safe for xen_xenstore.c. Stefan
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