Re: [PATCH v5 5/8] blockdev: Add a new IF type IF_OTHER

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Thu, Aug 04, 2022 at 05:30:40PM +0200, Markus Armbruster wrote:
>> Daniel P. Berrangé <berrange@redhat.com> writes:
>> 
>> > On Thu, Aug 04, 2022 at 04:56:15PM +0200, Markus Armbruster wrote:
>> >> Daniel P. Berrangé <berrange@redhat.com> writes:
>> >> 
>> >> > On Thu, Jul 28, 2022 at 10:46:35AM +0100, Peter Maydell wrote:
>> >> >> On Wed, 27 Jul 2022 at 20:03, Kevin Wolf <kwolf@redhat.com> wrote:
>> >> >> >
>> >> >> > Am 18.07.2022 um 11:49 hat Markus Armbruster geschrieben:
>> >> >> > > An OTP device isn't really a parallel flash, and neither are eFuses.
>> >> >> > > More fast-and-lose use of IF_PFLASH may exist in the tree, and maybe of
>> >> >> > > other interface types, too.
>> >> >> > >
>> >> >> > > This patch introduces IF_OTHER.  The patch after next uses it for an
>> >> >> > > EEPROM device.
>> >> >> > >
>> >> >> > > Do we want IF_OTHER?
>> >> >> >
>> >> >> > What would the semantics even be? Any block device that doesn't pick up
>> >> >> > a different category may pick up IF_OTHER backends?
>> >> >> >
>> >> >> > It certainly feels like a strange interface to ask for "other" disk and
>> >> >> > then getting as surprise what this other thing might be. It's
>> >> >> > essentially the same as having an explicit '-device other', and I
>> >> >> > suppose most people would find that strange.
>> >> >> >
>> >> >> > > If no, I guess we get to abuse IF_PFLASH some more.
>> >> >> > >
>> >> >> > > If yes, I guess we should use IF_PFLASH only for actual parallel flash
>> >> >> > > memory going forward.  Cleaning up existing abuse of IF_PFLASH may not
>> >> >> > > be worth the trouble, though.
>> >> >> > >
>> >> >> > > Thoughts?
>> >> >> >
>> >> >> > If the existing types aren't good enough (I don't have an opinion on
>> >> >> > whether IF_PFLASH is a good match), let's add a new one. But a specific
>> >> >> > new one, not just "other".
>> >> >> 
>> >> >> I think the common thread is "this isn't what anybody actually thinks
>> >> >> of as being a 'disk', but we would like to back it with a block device
>> >> >> anyway". That can cover a fair range of possibilities...
>> >> >
>> >> > Given that, do we even want/have to use -drive for this ?    We can use
>> >> > -blockdev for the backend and reference that from any -device we want
>> >> > to create, and leave -drive out of the picture entirely
>> >> 
>> >> -drive is our only means to configure onboard devices.
>> >> 
>> >> We've talked about better means a few times, but no conclusions.  I can
>> >> dig up pointers, if you're interested.
>> >
>> > For onboard pflash with x86, we've just got properties against the
>> > machine that we can point to a blockdev.
>> 
>> True, but the vast majority of onboard block devices doesn't come with
>> such properties.  Please see
>> 
>> Subject: On configuring onboard block devices with -blockdev (was: [PATCH v4 6/7] hw/nvram: Update at24c EEPROM init function in NPCM7xx boards)
>> Date: Mon, 15 Nov 2021 16:28:33 +0100
>> Message-ID: <875ystigke.fsf_-_@dusky.pond.sub.org>
>> https://lists.gnu.org/archive/html/qemu-devel/2021-11/msg03173.html
>
> My take away from your mail there is that in the absence of better ideas
> we should at least use machine properties for anything new we do so we
> don't make the problem worse than it already is. It feels more useful
> than inventing new IF_xxx possibilities for something we think is the
> wrong approach already.

The difficulty of providing machine properties for existing devices and
the lack of adoption even for new devices make me doubt they are a
viable path forward.  In the message I referenced above, I wrote:

    If "replace them all by machine properties" is the way forward, we
    need to get going.  At the current rate of one file a year (measured
    charitably), we'll be done around 2090, provided we don't add more
    (we've added quite a few since I did the first replacement).

I figure this has slipped to the 22nd century by now.

Yes, more uses of -drive are steps backward.  But they are trivially
easy, and also drops in the bucket.  Machine properties are more
difficult, and whether they actually take us forward seems doubtful.