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Date:   Wed, 18 Apr 2018 06:03:29 -0300
From:   Eduardo Habkost <ehabkost@redhat.com>
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
Subject: Re: [PATCH] KVM: X86: Allow userspace to define the microcode version
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On Wed, Apr 18, 2018 at 11:24:22AM +0800, Wanpeng Li wrote:
> 2018-04-18 4:24 GMT+08:00 Eduardo Habkost <ehabkost@redhat.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@redhat.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?

By looking at kvm_get_msr_feature(), it looks like
ioctl(system_fd, KVM_GET_MSRS) would return the host MSR value
for us.

-- 
Eduardo