Re: [Qemu-devel] Are -cdrom/-hda (or -drive if=ide) supposed to work in q35?

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 05.08.2014 um 11:13 hat Markus Armbruster geschrieben:
>> Kevin Wolf <address@hidden> writes:
>> 
>> > Am 01.08.2014 um 22:10 hat John Snow geschrieben:
>> >> 
>> >> On 06/12/2014 05:03 AM, Markus Armbruster wrote:
>> >> >Michael Tokarev <address@hidden> writes:
>> >> >
>> >> >>10.06.2014 10:34, Paolo Bonzini wrote:
>> >> >>>Il 10/06/2014 08:30, Michael Tokarev ha scritto:
>> >> >>>>Hello.
>> >> >>>>
>> >> >>>>The question is: are the drive shortcuts - -cdrom, -hda, -hdb etc -
>> >> >>>>supposed to work in -machine q35 too?  Or are they merely ignored?
>> >> >>>>
>> >> >>>>   qemu-system-x86_64 -machine q35 -cdrom foo.img
>> >> >>[]
>> >> >>>It should work.  I remember some complications due to AHCI not
>> >> >>>having slaves, but it is a bug.
>> >> >>It looks like the "short" -drive if=ide option does not connect the
>> >> >>created drive to any bus at all.  With the above command, or with
>> >> >>-drive if=ide,index=*,bus=*, info qtree does not show the drive at
>> >> >>all.  While -drive if=none,id=X -device ide-cd,drive=X connects the
>> >> >>drive to the right bus just fine.
>> >> >-drive mixes up configuration of backend and frontend (a.k.a. device
>> >> >  model), as follows:
>> >> >
>> >> >1. It always defines a backend.  "info block" shows them.
>> >> >
>> >> >2. It always defines a few frontend configuration bits for the device
>> >> >    models to pick up.
>> >> >
>> >> >3. Except with if=none, it posts a request to board code to create a
>> >> >    suitable frontend.  It's up to the board code to honor, reject or
>> >> >    ignore the request.  The i440FX boards honor it, the Q35 boards
>> >> >    ignore it.
>> >> >
>> >> >    Nobody has gotten around to making the Q35 boards honor it, in part
>> >> >    because there has been some confusion on what if=ide is supposed to
>> >> >    mean on Q35.  Should it connect an ide-hd / ide-cd in SATA mode or in
>> >> >    legacy PATA mode?
>> >> >
>> >> >    I've always argued for SATA, because for me if=ide does *not* imply a
>> >> >    specific kind of HBA any more than if=scsi does, and the "natural"
>> >> >    HBA for Q35 is AHCI in SATA mode.
>> >> >
>> >> >    Kevin (cc'ed) has argued for a way to make it connect in legacy PATA
>> >> >    mode.  I'd be fine with that, as long as it's off by default.
>> >> >
>> >> >    Patches welcome.
>> >> >
>> >> 
>> >> Kevin, (or anyone else with an opinion for that matter), what is the
>> >> reasoning behind wanting -cdrom to use the old PATA interfaces?
>> >
>> > The assumption that makes it a problem is that sooner or later we'll
>> > want to make Q35 the default. Most of the changes this brings in will
>> > make the guest see different, but generally still compatible hardware.
>> > AHCI however is not compatible with IDE in the sense that an OS that has
>> > only an IDE driver won't work with AHCI.
>> >
>> > It wouldn't be reasonable to break things like '-hda winxp.qcow2'. And
>> > in fact, if the internet is right, even newer Windows version give you
>> > trouble when you suddenly change from IDE to AHCI. So after an upgrade
>> > many users would find their existing guests to be broken.
>> 
>> I feel asking users of old guests to specify a machine type when the
>> default one no longer works is better than having defaults stuck in the
>> 90s forever.
>
> No matter whether IDE or AHCI, we do this for compatibility. If we
> wanted the best-performing default, no matter if it works for commonly
> used guests, we would make virtio the default.
>
> When I do something for compatibility, I generally try to make it work
> for as many cases as I can. You may disagree, AHCI is just a different
> tradeoff that you may like better. It provides less compatibility at a
> better performance. I don't however consider this improvment significant
> enough to break the default for a considerable number of guests.

Yes, it's a tradeoff.  Reasonable people can disagree on what to trade
and what to keep.

My take is that if you care for compatibility, specify the machine type
explicitly.  I'm unwilling to sacrifice much for other compatibility
scenarios.

>> That said...
>> 
>> >> For at least the immediate future, the AHCI device doesn't support
>> >> the mixed-mode SATA/PATA access models, though I suppose we could,
>> >> it seems like a more obvious and simple solution to just allow the
>> >> shorthand syntactic sugar commands to use the native bus of the
>> >> system until you specify otherwise.
>> >> 
>> >> I think I will probably begin writing a patch under this assumption
>> >> unless there is a better technical reason not to.
>> >
>> > I think the solution that was generally agreed on was to introduce a
>> > machine option that decides whether to provide an IDE or AHCI interface
>> > (similar to the BIOS option that commonly exists on real hardware). We
>> > just need someone to implement it.
>> 
>> ... if someone wants to implement legacy PATA for AHCI, I welcome
>> patches :)
>> 
>> However, I don't think we should keep if=ide broken on Q35 just to
>> "motivate" such work.
>
> I agree for the moment. It only becomes a strict requirement once we
> attempt to change the default machine type.

I agree that changing the default machine type is a separate issue.

> (And I think to get the terminology right, we have SATA hardware that
> can provide an IDE or AHCI software interface; not AHCI hardware that
> provides PATA/SATA.)

Okay :)