Re: [Qemu-devel] [PATCH v13 3/5] i386: Enable TOPOEXT feature on AMD EPYC CPU

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Jun 12, 2018 at 06:38:08PM +0000, Moger, Babu wrote:
[...]
> > I'm starting to think that enabling TOPOEXT automatically is
> > adding too much complexity and compatibility problems, and it's
> > better to leave this task to management software.
> > 
> > The main problem here is:
> > 
> > This works today with QEMU 2.12 + Linux <= 4.15:
> >   $ $QEMU -machine pc -cpu EPYC,enforce -smp
> > 8,sockets=2,cores=2,threads=2"
> > and must keep working with QEMU 3.0 and Linux <= 4.15.
> > 
> > In addition to that, the results for:
> >   $ $QEMU -machine pc-q35-3.0 -cpu EPYC,enforce [...]
> > must be deterministic and expose exactly the same CPUID data even
> > if host hardware or software changes, as long as the QEMU
> > command-line is the same.
> > 
> > Do you see a way to fulfill those two constraints while making
> > "-machine pc-q35-3.0 -cpu EPYC" enable TOPOEXT automatically?
> > 
> 
> Now(setting feature before x86_cpu_expand_features), enabling
> TOPOEXT appears to work fine.

What about the above constraints?  Are you really fulfilling
them?

This one is tricky:

] This works today with QEMU 2.12 + Linux <= 4.15:
]   $ $QEMU -machine pc -cpu EPYC,enforce -smp 8,sockets=2,cores=2,threads=2"
] and must keep working with QEMU 3.0 and Linux <= 4.15.

If we enable TOPOEXT unconditionally, the command-line won't work
with Linux <= 4.15.

If we enable TOPOEXT only if the kernel returns TOPOEXT on
GET_SUPPORTED_CPUID, we break the second constraint:

] The results for:
]   $ $QEMU -machine pc-q35-3.0 -cpu EPYC,enforce [...]
] must be deterministic and expose exactly the same CPUID data even
] if host hardware or software changes, as long as the QEMU
] command-line is the same.

-- 
Eduardo