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From:   Jim Mattson <jmattson@google.com>
Date:   Mon, 29 Jan 2018 14:49:51 -0800
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Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure
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And if we expect to introduce Cascade Lake into the pool in the
future, we use a Cascade Lake model number?

It sounds like you are suggesting that we set the model number to the
highest model number that will ever be introduced into the pool, at
any time in the future. That approach would also fail the
'is_skylake_era()' test. (Not to mention that we have no idea what
Intel's highest compatible model number will be.)

On Mon, Jan 29, 2018 at 2:41 PM, Andi Kleen <ak@linux.intel.com> wrote:
>> Even if we expose bit to indicate that FMS matches the underlying host, when does the guest know to query that?  The VM can be moved at any point in time, including after the guest asks if FMS matches host.
>
> There's no way to enable these mitigations later, so if you always
> have to enable the super set of all the mitigations for all the hosts you
> might be migrating too.
>
> As of currently that means if you want to ever migrate to Skylake you should
> set the Skylake model number and you're good.
>
> -Andi