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
