Re: [Qemu-devel] [PATCH v2 2/3] hw/sd: model a power-up delay, as a workaround for an EDK2 bug

From: Peter Maydell <peter.maydell@linaro.org>
To: Andrew Baumann <Andrew.Baumann@microsoft.com>
Cc: Igor Mitsyanko <i.mitsyanko@gmail.com>,
	Peter Crosthwaite <crosthwaitepeter@gmail.com>,
	"qemu-devel@nongnu.org Developers" <qemu-devel@nongnu.org>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Juan Quintela <quintela@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v2 2/3] hw/sd: model a power-up delay, as a workaround for an EDK2 bug
Date: Wed, 23 Dec 2015 19:20:49 +0000	[thread overview]
Message-ID: <CAFEAcA-W0UfnRtTCEG4WgJBMzw0hw=7+mZwPF0xot7dx+2rQQw@mail.gmail.com> (raw)
In-Reply-To: <BLUPR0301MB2034B77FDFAD79B62FA8FF009EE40@BLUPR0301MB2034.namprd03.prod.outlook.com>

On 21 December 2015 at 22:25, Andrew Baumann
<Andrew.Baumann@microsoft.com> wrote:

>> If you change the VMSTATE layout, you need to bump the version. As
>> this is very common code, it may have stricter version bump
>> requirements. Last I knew however, there was a way to add new fields
>> at the end of VMSD without breaking backwards compatibility. Peter or
>> Juan may know more.
>
> I'll admit that I didn't think about these issues when adding the
> fields, but after a (quick) look at vmstate_save_state() and
> vmstate_load_state(), they seem to be using named fields in a json
> format, so I don't think the order of fields should matter if we are
> adding new ones.

The on-the-wire format is not json, and order is important; field
names are not sent.

If we care about migration compatibility I think the recommendation
is to use subsections, rather than marking fields as only existing
in particular versions. (see docs/migration.txt for a discussion
of subsections).

thanks
-- PMM