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Re: [Qemu-devel] [QEMU RFC PATCH v3 4/6] Migration: migrate QTAILQ

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [QEMU RFC PATCH v3 4/6] Migration: migrate QTAILQ
Date: Tue, 7 Jun 2016 17:34:57 +0100
User-agent: Mutt/1.6.1 (2016-04-27)

* Paolo Bonzini (address@hidden) wrote:
> On 07/06/2016 16:43, Dr. David Alan Gilbert wrote:
> >   b) I think you should really try and split it into two parts:
> >     1) A VMSTATE_ARRAY_CUSTOM (?) - so it's an array of elements but we've 
> > got a special
> >        way of allocating/counting/reading the elements
> >     2) A version of that which is for a QTAILQ.
> >        It's important that whatever ends up on the migration stream doesn't 
> > reflect
> >        that it happens to be implemented as a QTAILQ; so if you decide to 
> > change
> >        it later the migration compatibility doesn't break.
> (Just to clarify before Jianjun wakes up) a VMSTATE_ARRAY is just a
> sequence of elements.  The count, if needed as in a VARRAY, is stored
> earlier and separately.  Currently lists (including this QTAILQ) are
> usually represented in the migration stream as a sequence of elements
> preceded by "1" and terminated by a "0".  Would you like to change it to
> a count + sequence as well?
> This would prevent using the new QTAILQ support for other existing lists
> such as virtio-blk and scsi's request lists.

That depends if it's something you're trying to keep migration compatibility
with;  if you're trying to keep format compaibility then sure keep it as is.
If you're not trying to keep compatibility, then why can't virtio-blk or
scsi request lists also use a count rather than the markers?

> >   c) Use new trace_ names for get_qtailq rather than misusing 
> > trace_vmstate_load_state*
> >   d) Add a trace_ for put_qtailq as well - that way when it goes horribly 
> > wrong we can
> >      turn the tracing on on both sides :-)
> >   e) Is there anyway to make QTAILQ_RAW_INSERT_TAIL any more readable?
> >      I don't think I understand why you can't use the standard QTAILQ 
> > macros.
> Because they assume a particular type. The "raw" version need to work on
> void*.

Ah OK.


> Thanks,
> Paolo
> >   f) You might need to fix up Amit's scripts/vmstate-static-checker.py
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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