Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode

[Daniel P . Berrangé]

On Wed, Aug 26, 2020 at 02:38:49PM +0200, Igor Mammedov wrote:
> On Fri, 21 Aug 2020 17:12:19 -0500
> Babu Moger <babu.moger@amd.com> wrote:
> 
> > To support some of the complex topology, we introduced EPYC mode apicid 
> > decode.
> > But, EPYC mode decode is running into problems. Also it can become quite a
> > maintenance problem in the future. So, it was decided to remove that code 
> > and
> > use the generic decode which works for majority of the topology. Most of the
> > SPECed configuration would work just fine. With some non-SPECed user inputs,
> > it will create some sub-optimal configuration.
> > Here is the discussion thread.
> > c0bcc1a6-1d84-a6e7-e468-d5b437c1b254@amd.com/">https://lore.kernel.org/qemu-devel/c0bcc1a6-1d84-a6e7-e468-d5b437c1b254@amd.com/
> > 
> > This series removes all the EPYC mode specific apicid changes and use the 
> > generic
> > apicid decode.
> 
> the main difference between EPYC and all other CPUs is that
> it requires numa configuration (it's not optional)
> so we need an extra patch on top of this series to enfoce that, i.e:
> 
>  if (epyc && !numa) 
>     error("EPYC cpu requires numa to be configured")

Please no. This will break 90% of current usage of the EPYC CPU in
real world QEMU deployments. That is way too user hostile to introduce
as a requirement.

Why do we need to force this ?  People have been successfuly using
EPYC CPUs without NUMA in QEMU for years now.

It might not match behaviour of bare metal silicon, but that hasn't
obviously caused the world to come crashing down.


Regards,
Daniel
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