Re: [Qemu-devel] Drop support for 32bit hosts in qemu? (was: [PULL 1/6] audio/hda: create millisecond timers that handle IO)

From: "Alex Bennée" <alex.bennee@linaro.org>
To: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>,
	Martin Schrodt <martin@schrodt.org>,
	Thomas Huth <thuth@redhat.com>, Max Reitz <mreitz@redhat.com>,
	qemu-devel@nongnu.org, Gerd Hoffmann <kraxel@redhat.com>
Subject: Re: [Qemu-devel] Drop support for 32bit hosts in qemu? (was: [PULL 1/6] audio/hda: create millisecond timers that handle IO)
Date: Wed, 27 Jun 2018 14:03:24 +0100	[thread overview]
Message-ID: <87d0wcwfdv.fsf@linaro.org> (raw)
In-Reply-To: <20180627110923.GG30628@redhat.com>

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

> On Wed, Jun 27, 2018 at 09:57:09AM +0200, Markus Armbruster wrote:
>> Thomas Huth <thuth@redhat.com> writes:
>>
>> > On 27.06.2018 08:51, Gerd Hoffmann wrote:
>> [...]
>> >> Drop support for 32bit hosts in qemu?
>> >
>> > I guess the only way to answer that question reliably is to send a patch
>> > to mark 32-bit hosts as deprecated...
>> >
>> > Anyway, you still have got to fix that problem with -m32 now somehow
>> > since we certainly can not drop 32-bit immediately.
>>
>> We certainly can if we want to.
>>
>> Our formal deprecation policy codifies our compromise between the need
>> to evolve QEMU and the need of its users for stable external interfaces.
>>
>> "Compiles on host X" is also a need, but it's a different one.
>> Evidence: "Supported build platforms" has its own appendix, separate
>> from "Deprecated features".  It's mum on 32-bit hosts.
>
> It is silent on host architecture coverage in general in fact. It was
> only really focusing on operating systems and so any arch coverage is
> at best inferred from what those OS target. Fedora covers 32-bit & 64-bit
> arches for example, so you could take that to imply we need to support
> both in QEMU.
>
> It is fuzzier though because while Peter has good OS coverage for his
> merge testing, I don't think he has full host arch coverage for everything
> that downstreams expect QEMU to build on ?

We do however have cross build infrastructure for all tcg/foo backends.
They are run by the shippable CI framework (which is how I noticed):

  https://app.shippable.com/github/qemu/qemu/runs?branchName=master

Once we have the changes made to the wiki I'd like to use:

  https://wiki.qemu.org/Template:CIStatus

In a few choice places on the wiki, including the front page, for better
visibility.

>> I'm not saying we *should* drop 32-bit hosts immediately.  Only that the
>> feature deprecation policy does not apply.
>>
>> Is QEMU still useful on 32-bit hosts?  Honest question!
>
> That answer varies depending on what you're using QEMU for I think. It
> could be that people see tools like qemu-img/qemu-nbd as useful even
> if they don't use system emulators. And of course userspace emulators
> are enough distinct use case.
>
> IMHO as long as distros are shipping 32-bit support, it is reasonable
> to assume there will be people who find QEMU useful to some degree.
>
> Personally I don't have any need for 32-bit hosts, but clearly some
> people do since otherwise distros would  have killed their 32-bit
> arches already.

I know of at least one kernel developer who uses qemu-system-aarch64 on
their 32 bit ARM Chromebook to test their 64 bit code. Next time I see
them I'll see if they are still using it.

My feeling is although probably not widespread it certainly can be handy.

>
> Regards,
> Daniel

--
Alex Bennée