Re: [Qemu-devel] [PATCH v5 1/6] machine: Convert the valid cpu types to use cpu_model

[Eduardo Habkost]

On Thu, Jan 23, 2020 at 07:48:21PM +0100, Philippe Mathieu-Daudé wrote:
> On 6/20/19 4:43 PM, Eduardo Habkost wrote:
> > On Thu, Jun 20, 2019 at 11:02:39AM +0200, Igor Mammedov wrote:
> > > On Tue, 18 Jun 2019 10:55:16 -0300
> > > Eduardo Habkost <address@hidden> wrote:
> > > 
> > > > On Tue, Jun 18, 2019 at 01:34:10PM +0200, Igor Mammedov wrote:
> > > > > On Mon, 17 Jun 2019 13:27:00 -0300
> > > > > Eduardo Habkost <address@hidden> wrote:
> > > > > > On Mon, Jun 17, 2019 at 05:33:43PM +0200, Igor Mammedov wrote:
> > > > > > > On Mon, 17 Jun 2019 17:15:21 +0200
> > > > > > > Philippe Mathieu-Daudé <address@hidden> wrote:
> > > > > > [...]
> > > > > > > > Yes. Eduardo and you should write some lines to explain this, 
> > > > > > > > and then
> > > > > > > > we will follow :)
> > > > > > > Unfortunately I don't recall details anymore. One could check out 
> > > > > > > all
> > > > > > > implementations of class_by_name callbacks to find out current 
> > > > > > > state.
> > > > > > 
> > > > > > See this message for a summary of existing class_by_name quirks:
> > > > > > 
> > > > > >    https://www.mail-archive.com/address@hidden/msg615503.html
> > > > > >    Date: Wed, 08 May 2019 10:34:44 +0200
> > > > > >    Message-ID: <address@hidden>
> > > > > >    Subject: Re: [Qemu-devel] [PATCH 0/7] Delete 16 
> > > > > > *_cpu_class_by_name() functions
> > > > > > 
> > > > > > I'm unsure about Igor's suggestion to get rid of CPU model names
> > > > > > and use only QOM type names in external interfaces.  In either
> > > > > > case, we can still simplify the rules rules and reduce the amount
> > > > > > of arch-specific code.
> > > > > as far as we have cpu_class_by_name, we have to watch over that
> > > > > new patches/targets won't add some custom handling/fallbac/whatnot.
> > > > 
> > > > We can get rid of CPUClass::cpu_class_by_name() without changing
> > > > the external interfaces provided by QEMU.
> > > if you mean QMP, than it is possible to keep model there where
> > > it already exists. Based on experiment [1](x86) I did, it's local to
> > > affected target and doesn't pollute other code.
> > 
> > I mean both command line and QMP.
> > 
> > > 
> > > > I don't have a strong opinion about using only QOM types at -cpu,
> > > > yet.  But first we need to get rid of the arch-specific CPU model
> > > > name exceptions enumerated at the URL above (which would be very
> > > > welcome).
> > > One way to get rid of them is to deprecate them in favor of strict
> > > match (no fallback/substitutions/aliases) to typename and when we
> > > drop it make switch type based naming only.
> > 
> > This is true, but is it desirable?  Aren't we just moving
> > complexity elsewhere?  Management software would still need to
> > translate CPU models from existing VM configurations to QOM type
> > names.
> > 
> > > 
> > > 1) I've just took a quick look at how much of duplicated/confusing
> > > code we could get rid off if we drop cpu_class_by_name/*_cpu_list.
> > > It amounts to >800LOC of trivial removal (not counting ppc/s390
> > > that depend on model naming heavily and in need of some non
> > > trivial refactoring)
> > 
> > Removing the code might be trivial.  Coordinating with the
> > maintainers of all architectures to confirm this is really
> > something everybody wants is the difficult part.
> > 
> > If you really want to do it, please make sure all the
> > architecture maintainers (and libvirt developers) are involved in
> > the discussion.
> 
> From the previous link (summary of existing class_by_name quirks):
> https://www.mail-archive.com/address@hidden/msg615737.html
> 
>   "Deprecating unwanted stuff now is likely to make a
>    later cleanup so much easier."
> 
> This was 8 months ago :/
> 
> IIUC we need to restart this topic addressing the libvirt community first,
> then see with the QEMU one?

The next step is to deprecate the weird and pointless exceptions
listed at the URL you mentioned.  Not all exceptions will affect libvirt (maybe 
most won't).

In either case, we can just use qemu-deprecated.texi to
communicate this to libvirt developers.  MAINTAINERS already has
a "R: libvir-list" line for qemu-deprecated.texi.

-- 
Eduardo