Re: [PATCH 0/4] KVM: x86: kvm_mwait_in_guest() cleanup and fixes

From: Gabriel L. Somlo
Date: Sat May 06 2017 - 12:48:51 EST

On Thu, May 04, 2017 at 08:07:15PM +0200, Radim KrÄmÃÅ wrote:
> 2017-05-04 13:56-0400, Gabriel L. Somlo:
> > If I wanted to test this (e.g. with OS X 10.8 guests on several of my older
> > Mac boxes running Fedora), which git repo would you have me use? (The series
> > won't apply directly on top of git://
> The queue branch of that repo. This series depends on a patch that is
> applied there:
> 668fffa3f838 kvm: better MWAIT emulation for guests
> I forgot to mention that, sorry.

OK, here's where I'm at right now:

With this series applied on top of 'queue', my MacbookAir4,2 running
F25 (with the kvm/queue kernel) works fine, i.e. loads the kvm-intel
module successfully, and mwaits in L1 guest mode, reporting 400% cpu
but staying cool (guest started with -smp 4).

So far, so good.

On the MacPro1,1, I first had to revert 2c82878b0cb38fd
("KVM: VMX: require virtual NMI support") to get around this error:

# modprobe -v kvm-intel
insmod /lib/modules/4.11.0-rc3+/kernel/virt/lib/irqbypass.ko
insmod /lib/modules/4.11.0-rc3+/kernel/arch/x86/kvm/kvm.ko
insmod /lib/modules/4.11.0-rc3+/kernel/arch/x86/kvm/kvm-intel.ko
modprobe: ERROR: could not insert 'kvm_intel': Input/output error

Next, it turns out that on the MacPro1,1 kvm_mwait_in_guest() returns
TRUE, which causes OS X 10.7 (the one that mwaits without checking
CPUID) to misbehave. Forcing the function to return 0 (FALSE) solves
the problem:

diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index b49add7..249362c 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -216,9 +216,12 @@ static inline u64 nsec_to_cycles(struct kvm_vcpu *vcpu, u64 nsec)

static inline bool kvm_mwait_in_guest(void)
- return boot_cpu_has(X86_FEATURE_MWAIT) &&
+ bool ret;
+ ret = boot_cpu_has(X86_FEATURE_MWAIT) &&
!boot_cpu_has_bug(X86_BUG_AMD_E400) &&
+ printk(KERN_INFO "kvm_mwait_in_guest: %d\n", ret);
+ return 0;


After this change, I get:

[ 1201.529002] kvm_mwait_in_guest: 1
[ 1201.529024] kvm_mwait_in_guest: 1
[ 1201.529029] kvm_mwait_in_guest: 1
[ 1201.529038] kvm_mwait_in_guest: 1
[ 1201.529047] kvm_mwait_in_guest: 1
[ 1225.150235] kvm: MONITOR instruction emulated as NOP!
[ 1225.150240] kvm: MWAIT instruction emulated as NOP!

indicating that it *would* have returned TRUE if I let it :)

This is a 2x dual-core Xeon, cca 2006 vintage, and the last (4th) CPU
in /proc/cpuinfo returns:

processor : 3
vendor_id : GenuineIntel
cpu family : 6
model : 15
model name : Intel(R) Xeon(R) CPU 5150 @ 2.66GHz
stepping : 6
microcode : 0xd2
cpu MHz : 2659.977
cache size : 4096 KB
physical id : 3
siblings : 2
core id : 0
cpu cores : 2
apicid : 6
initial apicid : 6
fpu : yes
fpu_exception : yes
cpuid level : 10
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx lm constant_tsc arch_perfmon pebs bts rep_good nopl cpuid aperfmperf pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm dca lahf_lm tpr_shadow dtherm
bugs :
bogomips : 5320.03
clflush size : 64
cache_alignment : 64
address sizes : 36 bits physical, 48 bits virtual
power management:

So, in conclusion; it's not important to *me* that this old machine
keeps working, I'm just volunteering test data points. So please don't
feel obligated in any way to go out of your way on my account. OTOH,
I'm happy to provide feedback as long as you would like me to.

Along the same lines: Paolo, as the author of commit 2c82878b0cb38fd,
is the Xeon chip listed above one of the "obsolete for virtualization"
models ? In that case, it makes no sense for me to keep using it for
tests, and the fact that it misbehaves with L1 MWAIT should also not
matter at all.

Let me know what you all think.