Re: [libvirt] [Qemu-devel] [Question] why x2apic's set by default without host support(on Nehalem CPU).
On Mon, Jul 22, 2013 at 07:24:24PM +0800, Peter Huang(Peng) wrote:
Hi, Everyone
I have been encountered a problem recently. My box uses Nehalem E5520 CPU, and this model doesn't support x2apic feature. I created a VM on KVM, and uses cpu mode=pass-through, I can get the right cpu model from the VM, but I can also see that the CPU in the vm supports x2apic.
This seems werid. From QEMU's CPU model definition, Nehalem also doesn't support x2apic.
Can anyone explain it?
libvirt's "host-passthrough" uses "-cpu host', and it "-cpu host" enables every feature that can be enabled on the host. As KVM x2apic emulation doesn't require host CPU support, it gets enabled. If the behavior of "-cpu host" doesn't match libvirt expectations, we need to clarify what are the requirements, and maybe have a "try to be close to host CPU mode" as opposed to the current "enable everything that can be enabled" mode. -- Eduardo
libvirt's "host-passthrough" uses "-cpu host', and it "-cpu host" enables every feature that can be enabled on the host. From my test results, I found that even when use host-passthrough mode, VM's cpu features are very different from host, this doesn't match what host-passthrough mode's explanation.
libvirt's option exlanation: With this mode, the CPU visible to the guest should be exactly the same as the host CPU even in the aspects that libvirt does not understand.
If the behavior of "-cpu host" doesn't match libvirt expectations, we need to clarify what are the requirements, and maybe have a "try to be close to host CPU mode" as opposed to the current "enable everything that can be enabled" mode. Does the results above indicates what you mean to do? or the current way doesn't work as we expect.
Thanks!
On Tue, Jul 23, 2013 at 10:44:48 +0800, Peter Huang(Peng) wrote:
libvirt's "host-passthrough" uses "-cpu host', and it "-cpu host" enables every feature that can be enabled on the host. From my test results, I found that even when use host-passthrough mode, VM's cpu features are very different from host, this doesn't match what host-passthrough mode's explanation.
libvirt's option exlanation: With this mode, the CPU visible to the guest should be exactly the same as the host CPU even in the aspects that libvirt does not understand.
The libvirt documentation is what needs to be updated. While host-passthrough is asking for a CPU which is as close as possible to the real host CPU, there are features that need special handling before they can be provided to a guest. And if the hypervisor does not provide that handling, it may just filter such feature out. Also if some features can be efficiently provided to a guest even though the host CPU does not provide them (x2apic is an example of such feature), they may be provided to a guest. Jirka
Hi,Jiri Thanks for replying. So why VM identified different features from host is due to the hypervisor filtering. I will dig into the hypervisor of how it filter features, and update status later. Thanks again. On 2013-07-23 19:27, Jiri Denemark wrote:
libvirt's "host-passthrough" uses "-cpu host', and it "-cpu host" enables every feature that can be enabled on the host. From my test results, I found that even when use host-passthrough mode, VM's cpu features are very different from host, this doesn't match what host-passthrough mode's explanation.
libvirt's option exlanation: With this mode, the CPU visible to the guest should be exactly the same as the host CPU even in the aspects that libvirt does not understand. The libvirt documentation is what needs to be updated. While host-passthrough is asking for a CPU which is as close as possible to
On Tue, Jul 23, 2013 at 10:44:48 +0800, Peter Huang(Peng) wrote: the real host CPU, there are features that need special handling before they can be provided to a guest. And if the hypervisor does not provide that handling, it may just filter such feature out. Also if some features can be efficiently provided to a guest even though the host CPU does not provide them (x2apic is an example of such feature), they may be provided to a guest.
Jirka
FYI, just for information sharing. KVM emulated x2apic feature no matter host supports it or not. You can get the code from arch/x86/kvm/cpuid.c. kvm_dev_ioctl_get_supported_cpuid ()->do_cpuid_ent() /* we support x2apic emulation even if host does not support * it since we emulate x2apic in software */ entry->ecx |= F(*X2APIC*); QEMU will get this feature through kvm_ioctl(). On 2013-07-24 8:48, Peter Huang(Peng) wrote:
Hi,Jiri
Thanks for replying.
So why VM identified different features from host is due to the hypervisor filtering. I will dig into the hypervisor of how it filter features, and update status later.
Thanks again.
participants (3)
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Eduardo Habkost -
Jiri Denemark -
Peter Huang(Peng)