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

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Message-ID: <CANRm+CwA22VmpA5cfQg8mF_Xke_hXtP+_ui5xU+LV8v0JNj=3Q@mail.gmail.com>
Date:   Wed, 18 Apr 2018 11:24:22 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Eduardo Habkost <ehabkost@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...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

2018-04-18 4:24 GMT+08:00 Eduardo Habkost <ehabkost@...hat.com>:
> On Tue, Apr 17, 2018 at 06:40:58PM +0800, Wanpeng Li wrote:
>> Cc Eduardo,
>> 2018-02-26 20:41 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>> > 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.
>>
>> Hi Paolo,
>>
>> Do you mean the host admin to get the ucode_rev from the host and set
>> to -cpu host, ucode_rev=xxxxxx or qemu get the ucode_rev directly by
>> rdmsr?
>
> QEMU setting ucode_rev automatically using the host value when
> using "-cpu host" (with no need for explicit ucode_rev option)
> makes sense to me.

QEMU can't get the host value by rdmsr MSR_IA32_UCODE_REV directly
since rdmsr will #GP when ring !=0, any idea?

Regards,
Wanpeng Li