Re: [Qemu-devel] [PATCH RFC] i386: Add new Hygon 'Dhyana' and 'Dhyana-IBPB' CPU models

[Daniel P . Berrangé]

On Fri, Apr 12, 2019 at 08:37:49PM +0800, Pu Wen wrote:
> Add two new base CPU models called 'Dhyana' and 'Dhyana-IBPB' to model
> processors from Hygon Dhyana (family 18h), which derived from AMD EPYC
> (family 17h). 'Dhyana-IBPB' is a copy of the 'Dhyana' CPU model with
> just CPUID_8000_0008_EBX_IBPB added.

Do we really need to support both CPU variants ?

The IBPB feature is the Spectre fix which dates from January 2018.
Having both made (limited) sense for EPYC as EPYC CPUs existed
already, so there would be a mix of EPYC CPUs with and without the
microcode that adds IBPB.

The Google results I see suggest that Dhyana CPUs started to become
available in July 2018, so surely they would come with microcode
that has the IBPB right from day 1 ?

Am I missing anything that means there will be a compelling need
to support Dhyana /without/ IBPB ?

If not, then we only need 1 CPU model and that would include IBPB.

Regards,
Daniel
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