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

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Date:   Wed, 4 Jul 2018 18:21:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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 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.

Paolo