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

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Message-ID: <20180423132347.GE22238@pd.tnic>
Date:   Mon, 23 Apr 2018 15:23:47 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Eduardo Habkost <ehabkost@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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 Mon, Apr 23, 2018 at 10:08:23AM -0300, Eduardo Habkost wrote:
> On Mon, Apr 23, 2018 at 02:58:49PM +0200, Borislav Petkov wrote:
> > On Wed, Apr 18, 2018 at 12:36:37PM +0200, Paolo Bonzini wrote:
> > > On 18/04/2018 11:03, Eduardo Habkost wrote:
> > > >>> 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.
> > > 
> > > Yes, that's exactly what it was introduced for (together with other MSRs
> > > including VMX capabilities).
> > 
> > Can't qemu do:
> > 
> > grep microcode /proc/cpuinfo | awk '{ print $3 }' | head -n 1
> 
> It could, but why would QEMU do it if a real API already exists
> for that?

To save an MSR read per logical CPU but from the looks of it, that
KVM_GET_MSRS is widely used in qemu with kvm_get_msrs() so I guess one
more MSR doesn't matter.

-- 
Regards/Gruss,
    Boris.

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