linux-kernel - Re: [PATCH v2] KVM: Expose the split lock detection feature to guest VM

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Date:   Wed, 4 Jul 2018 20:39:50 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>
cc:     Jingqi Liu <jingqi.liu@...el.com>, rkrcmar@...hat.com,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        wei.w.wang@...el.com, Robert Hoo <robert.hu@...ux.intel.com>
Subject: Re: [PATCH v2] KVM: Expose the split lock detection feature to guest
 VM

On Wed, 4 Jul 2018, Paolo Bonzini wrote:
> On 04/07/2018 16:51, Thomas Gleixner wrote:
> > There is no rush for this to be in KVM/QEMU now because all what exists for
> > this new split lock thing is 'silicon' running on an emulator. And w/o
> > support in the kernel proper this is completely useless.
> 
> That's good.  I assumed it was IceLake, in which case the feature would
> block the definition of a standard IceLake CPU model in QEMU.
> 
> > So this needs the following things:
> > 
> >   1) Proper enumeration via CPUID or MISC_FEATURES. The current detection
> >      hack is just broken.
> 
> Yes please.
> 
> >   2) A proper host side implementation, which then automatically makes the
> >      stuff usable in a guest once it is exposed.
> 
> If the CPUID bit or MISC_FEATURES is added, you don't even need the host
> side for the guests to use it.  It's only needed now because of the ugly
> MSR-based detection.
> 
> >   3) A proper way how to expose MSR_TEST_CTL to the guest, but surely not
> >      with extra split_lock_ctrl voodoo. It's an MSR nothing else. KVM/QEMU
> >      have standartized ways to deal with MSRs and the required selective
> >      bitwise access control.
> 
> That part is pretty much standard, I'm not worried about it.  We have
> one variable in struct kvm_vcpu_arch for each MSR (or set of MSRs) that
> we expose, so that's the split_lock_ctrl voodoo. :)
> 
> Once the detection is sorted out, KVM is easy.

That's what I thought and even if it was IceLeak then they still can flip a
CPUID/MISC FEATURE bit in their binary BIOS blob or ucode. We really need
to push back hard on these half baken features which need voodoo
programming to detect.

Thanks,

	tglx