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

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Date:   Mon, 26 Feb 2018 13:41:38 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        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 26/02/2018 13:22, Borislav Petkov wrote:
> On Mon, Feb 26, 2018 at 01:18:07PM +0100, Paolo Bonzini wrote:
>>> In this context, "host-initiated" write means written by KVM userspace
>>> with ioctl(KVM_SET_MSR).  It generally happens only on VM startup, reset
>>> or live migration.
>>
>> To be clear, the target of the write is still the vCPU's emulated MSR.
> 
> So how am I to imagine this as a user:
> 
> qemu-system-x86_64 --microcode-revision=0xdeadbeef...

More like "-cpu foo,ucode_rev=0xdeadbeef".  But in practice what would
happen is one of the following:

1) "-cpu host" sets ucode_rev to the same value of the host, everyone
else leaves it to zero as is now.

2) Only Amazon uses this feature and we ignore it. :)

Paolo