Re: [Qemu-devel] [PATCH] blockdev: reset werror/rerror on drive_del

[Paolo Bonzini]

Il 04/06/2013 18:37, Markus Armbruster ha scritto:
> I figure the bit that can't be resolved now is letting the user switch
> off "stop on error" safely before a drive_del.  Even if we had a command
> for that, there'd still be a window between that command's execution and
> drive_del's.  Your patch solves the problem by having drive_del switch
> it off unconditionally.  Oookay, but please document it, because it's
> not exactly obvious.

It is not obvious, but it is not surprising either when you see it (i.e.
you won't really be surprised by the errors in the guest and won't need
to know that, under the hood, rerror has been changed from the value you
specified).

> Re "the guest gets the ENOMEDIUM error": depends on the device.  I doubt
> disks can signal "no medium", and even if they could, I doubt device
> drivers are prepared for it.

SCSI disks can signal whatever they want.  Device drivers will just
treat it as any other error (sense code) they don't recognize.

> Re "this scenario actually happens sometimes during libvirt disk hot
> unplug, where device_del is followed by drive_del": if I remember
> correctly, libvirt disk hot unplug runs drive_del right after
> device_del, opening a window where the guest sees a dead device.  That's
> asking for trouble, and trouble is known to oblige.

I think it's causing too much trouble though, and Stefan's patch is
making the trouble evident to the guest.  Surprise removal is a fact of
life, I don't think it makes sense to stop the machine on surprise
removal.  It's very different from I/O errors.

Paolo