How To Enable The Svm Cpu Flag Inside A Vm?

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I’m trying to get the Android Emulator to run inside a kvm vm on CentOS-6. Apparently the latest Android Emulators cannot run without hardware acceleration so I am trying to get the vm to see the svm cpu flag.

$ grep model\ name /proc/cpuinfo | sort -u model name : AMD Phenom(tm) II X4 965 Processor

$ grep svm /proc/cpuinfo | sort -u flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm 3dnowext 3dnow constant_tsc rep_good nonstop_tsc extd_apicid pni monitor cx16 popcnt lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt npt lbrv svm_lock nrip_save

$ cat /sys/module/kvm_amd/parameters/nested

I have tried copying the cpu options (Opteron_G3) in the host cpu configuration and setting the svm flag to either “model”, “required” or
“force” to no avail:

$ ps waux | grep qemu qemu 7962 7.2 21.9 3722092 2680668 ? Rl 13:23
6:36 /usr/libexec/qemu-kvm -name C7-Android -S -M rhel6.6.0 -cpu Opteron_G3,+invtsc,+wdt,+skinit,+ibs,+osvw,+3dnowprefetch,+cr8legacy,
+vme -enable-kvm -m 3072 -realtime mlock=off -smp

So the question is: How do I enable the svm flag in the guest?

Regards, Leonard.

7 thoughts on - How To Enable The Svm Cpu Flag Inside A Vm?

  • Hi,

    The qemu-kvm option “-enable-nesting” that I dug up in the source looks promising. No mention in the man page on either C6 or C7 but it is mentioned here:

    How do I pass the “-enable-nesting” option to qemu-kvm using either, virsh or the domain xml?

    Hoping to avoid having to patch the source a such:

    — qemu-kvm- 2009-12-29 21:46:34.000000000 +0100
    +++ qemu-kvm- 2016-09-25 16:35:02.984334623 +0200
    @@ -1811,8 +1811,10 @@ void cpu_x86_cpuid(CPUX86State *env, uin
    /* disable CPU features that KVM cannot support */

    /* svm */
    if (!kvm_nested)
    *ecx &= ~CPUID_EXT3_SVM;
    /* 3dnow */
    *edx &= ~0xc0000000;
    } else {

    Regards, Leonard.

  • Hello Jonathan,

    Right. The fact that this has been lingering in the source for years on end and only now is being announced as a tech preview seems to answer my question in the sense that “no this is not possible” :S .

    Guess I’ll have to fire up my build vm then and see if nuking that test for “kvm_nested” allows me to pass the svm cpu flag from a C6 host.

    Regards, Leonard.

  • Hi,

    Gave the beta a try. The option “nested” for AMD cpu’s is no longer automatically set. You have to either add kvm-amd.nested=1 on the kernel boot line or configure modprobe:

    echo “options kvm-amd nested=1” >> /etc/modprobe.d/kvm_amd.conf

    Configure the guest to use “host-passthrough” as the cpu type. Using
    “host-model” (copy host cpu) did not work for me. /dev/kvm wasn’t available.

    I haven’t installed Android Studio yet, but since both the svm cpu flag and /dev/kvm seem to be available I expect the Android Emulator to work inside the vm.

    Enjoy, Leonard.

  • Hello Barak,

    Thanks for pointing this out. It seems I do not have to wait for the release of 7.3 to get this to work.

    Using “host-passthrough” as the vm cpu type after enabling nested virtualization for my AMD Opteron on a pristine CentOS-7.2 system indeed passes the svm cpu flag and /dev/kvm to the guest. I haven’t actually installed Android Studio yet, but I am quite confident the emulator will work. This is a compelling reason to update my desktop from C6 to C7. Hadn’t planned on that for a few more years but oh well… :)

    Regards, Leonard.