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Date: Wed, 21 Jun 2017 14:48:22 -0300
From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: David Hildenbrand <david@redhat.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        rkrcmar@redhat.com, dvyukov@google.com,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC] KVM: VMX: drop vmm_exclusive module parameter
Message-ID: <20170621174822.GR13640@kernel.org>
References: <20170310114713.7571-1-david@redhat.com>
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Em Fri, Mar 10, 2017 at 12:47:13PM +0100, David Hildenbrand escreveu:
> vmm_exclusive=0 leads to KVM setting X86_CR4_VMXE always and calling
> VMXON only when the vcpu is loaded. X86_CR4_VMXE is used as an
> indication in cpu_emergency_vmxoff() (called on kdump) if VMXOFF has to be
> called. This is obviously not the case if both are used independtly.
> Calling VMXOFF without a previous VMXON will result in an exception.
> 
> In addition, X86_CR4_VMXE is used as a mean to test if VMX is already in
> use by another VMM in hardware_enable(). So there can't really be
> co-existance. If the other VMM is prepared for co-existance and does a
> similar check, only one VMM can exist. If the other VMM is not prepared
> and blindly sets/clears X86_CR4_VMXE, we will get inconsistencies with
> X86_CR4_VMXE.
> 
> As we also had bug reports related to clearing of vmcs with vmm_exclusive=0
> this seems to be pretty much untested. So let's better drop it.
> 
> While at it, directly move setting/clearing X86_CR4_VMXE into
> kvm_cpu_vmxon/off.

Oh well, I was using, as suggested by Alexander, this parameter to be
able to use Intel PT on the host on a Broadwell machine, i.e.:

  perf record -e intel_pt// usleep 1
  perf script

would show decoded Intel PT records, no more :-\ But I'm clueless about
KVM internals, so just reporting the change in behaviour for this very
specific use case.

Now I don't know if this is something that would make Intel PT be usable
on Broadwell machines but wouldn't be required with newer chips, will
test with a Kaby Lake i5 7500 when back at my home office...

- Arnaldo