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Re: [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine |
Date: |
Mon, 4 Jun 2018 20:17:23 +0300 |
On Mon, Jun 04, 2018 at 10:26:24AM -0300, Eduardo Habkost wrote:
> On Mon, Jun 04, 2018 at 02:01:29PM +0100, Daniel P. Berrangé wrote:
> > On Mon, Jun 04, 2018 at 09:54:15AM -0300, Eduardo Habkost wrote:
> > > On Mon, Jun 04, 2018 at 04:38:22AM +0300, Michael S. Tsirkin wrote:
> > > > On Sun, Jun 03, 2018 at 12:27:49PM +0300, Marcel Apfelbaum wrote:
> > > > > Moving to QEMU 3.0 seems like a good opportunity for such a change.
> > > > >
> > > > > I440FX is really old and does not support modern features like IOMMU.
> > > > > Q35's SATA emulation is faster than pc's IDE, native PCI express
> > > > > hotplug
> > > > > is cleaner than ACPI based one and so on...
> > > > >
> > > > > Also the libvirt guys added very good support for the Q35 machine
> > > > > (thanks!).
> > > > >
> > > > > Management software should always specify the machine type and for the
> > > > > current setups, adding '-machine pc' to the command line is not such a
> > > > > big deal.
> > > > >
> > > > > In time the pc machine will fade out and we will probably stop adding
> > > > > new versions at some point.
> > > > >
> > > > > Signed-off-by: Marcel Apfelbaum <address@hidden>
> > > >
> > > > For command line users, I think changing the default isn't nice.
> > > >
> > > > Yes it's easy to add -machine pc but there's no documentation
> > > > that tells you to do so. Add to that shortcuts like -cdrom
> > > > stop working, hotplug needs extra bridges to work, and one
> > > > can see that while management tool users benefit from q35,
> > > > command line users will suffer.
> > > >
> > > > Can't we add a tag for management without changing the command line
> > > > default? How about "management-default"? "recommended"? "latest"?
> > >
> > > We could add new aliases if they are useful for management
> > > software, but we would need a well-defined use case and set of
> > > requirements+expectations for the new alias.
> >
> > I'm not convinced by the idea of adding a distinct default "for mgmt". All
> > the problems described wrt 'q35' vs 'pc' apply equally to management apps
> > as they do to humans. It just happens that one common mgmt layer (libvirt)
> > knows how to handle some of the complexity of q35. Other mgmt apps though
> > are just as likely to be hurt by the change as humans are. So effectively
> > the proposed "for mgmt" is actually "for libvirt >= some version", which
> > feels like a layering violation to me.
>
> This means the new alias would be used only if requested
> explicitly by management software (not used automatically by
> libvirt).
>
> Taking that into account, I still don't see what exactly would be
> the use case here, and what exactly users can/can't expect when
> using the new alias.
Let's see what we have now first:
1. We have a requirement for the user to save the machine type on install
and maintain it with the image (a separate thread discusses saving that
as part of a qcow2 image).
2. If you use an alias instead you are supposed to resolve it
and save the resolved value. If you save the alias instead,
you can't do cross-version live migration.
3. If you don't specify anything you get a machine tagged default. You are
supposed to find it and save the value found. If you don't and just
keep using the default, you can't do cross-version live migration.
---
So now we would like to relax 3 to say
"If you don't and just keep using the default, some images might not
boot".
unfortunately we probably can't change 3 like this.
So what I propose instead is simply:
4. If you find a machine type value tagged "qmp-default" you must save
the value found. If you don't and just keep using the qmp-default each
time, then existing guest images won't boot.
This relaxed compatibility requirement allows for advanced features
as compared to default.
> --
> Eduardo
- [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine, Marcel Apfelbaum, 2018/06/03
- Re: [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine, Michael S. Tsirkin, 2018/06/04
- Re: [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine, Daniel P . Berrangé, 2018/06/04
- Re: [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine, Marcel Apfelbaum, 2018/06/04
- Re: [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine, Eduardo Habkost, 2018/06/04
Re: [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine, Marcel Apfelbaum, 2018/06/04