linux-kernel - Re: [PATCH] KVM: X86: Allow userspace to define the microcode version

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Message-ID: <2944C5CC-A3AF-4B33-B03F-7EC28FC351CB@oracle.com>
Date:   Tue, 24 Apr 2018 09:44:15 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>
CC:     Eduardo Habkost <ehabkost@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: X86: Allow userspace to define the microcode version

On April 24, 2018 1:09:00 AM EDT, Paolo Bonzini <pbonzini@...hat.com> wrote:
>On 24/04/2018 05:14, Konrad Rzeszutek Wilk wrote:
>> You would need to include the microcode version in the migration
>stream.
>> 
>> But this brings another point - what if we want to manifest certain
>> new CPUID bits?
>
>You don't do that across migration.  Generally if you want to do live
>migration and you set up the guest to know everything about the host
>(down to the microcode level), you should make sure your host are
>pretty
>much identical.

I understand how it ought to be but sadly the cloud vendors have a mix of hardware.


With the retpoline/IBRS support (like what RH kernel has) you could migrate from Skylake to Broadwell and switching over from IBRS to retpoline would be good.

Hence asking about this.

>
>Paolo