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Re: [PATCH v11 1/2] docs: improve qcow2 spec about extending image heade

From: Eric Blake
Subject: Re: [PATCH v11 1/2] docs: improve qcow2 spec about extending image header
Date: Fri, 31 Jan 2020 08:46:48 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 1/31/20 8:22 AM, Vladimir Sementsov-Ogievskiy wrote:
Make it more obvious how to add new fields to the version 3 header and
how to interpret them.

The specification is adjusted so that for new defined optional fields:

1. Software may support some of these optional fields and ignore the
    others, which means that features may be backported to downstream
    Qemu independently.
2. If we want to add incompatible field (or a field, for which some its

s/some/some of/

    values would be incompatible), it must be accompanied by
    incompatible feature bit.

Also the concept of "default is zero" is clarified, as it's strange to
say that the value of the field is assumed to be zero for the software
version which don't know about the field at all and don't know how to
treat it be it zero or not.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
  docs/interop/qcow2.txt | 45 +++++++++++++++++++++++++++++++++++++++---
  1 file changed, 42 insertions(+), 3 deletions(-)
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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