Re: [PATCH 2/4] kvm: Allow invtsc migration if tsc-khz is set explicitly

Marcelo Tosatti
2017-01-04T11:46:01Z

On Tue, Dec 27, 2016 at 05:21:18PM -0200, Eduardo Habkost wrote:
> We can safely allow a VM to be migrated with invtsc enabled if
> tsc-khz is set explicitly, because QEMU already refuses to start
> if it can't set the TSC frequency to the configured value.

On the source host, yes, but on the destination it does not.

SOURCE: -cpu kvm64,tsc-frequency=2660001000
DESTINATION: -cpu kvm64

Only prints:

qemu-system-x86_64: warning: TSC frequency mismatch between VM (2660001
kHz) and host (2893428 kHz), and TSC scaling unavailable
qemu-system-x86_64: warning: TSC frequency mismatch between VM (2660001
kHz) and host (2893428 kHz), and TSC scaling unavailable

And allows migration.

> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> ---
> target/i386/kvm.c | 19 ++++++++++---------
> 1 file changed, 10 insertions(+), 9 deletions(-)
>
> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> index a26290f..6a51399 100644
> --- a/target/i386/kvm.c
> +++ b/target/i386/kvm.c
> @@ -962,15 +962,16 @@ int kvm_arch_init_vcpu(CPUState *cs)
> has_msr_mcg_ext_ctl = has_msr_feature_control = true;
> }
>
> - if ((env->features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC) &&
> - invtsc_mig_blocker == NULL) {
> - /* for migration */
> - error_setg(&invtsc_mig_blocker,
> - "State blocked by non-migratable CPU device"
> - " (invtsc flag)");
> - migrate_add_blocker(invtsc_mig_blocker);
> - /* for savevm */
> - vmstate_x86_cpu.unmigratable = 1;
> + if (!env->user_tsc_khz) {
> + if ((env->features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC) &&
> + invtsc_mig_blocker == NULL) {
> + /* for migration */
> + error_setg(&invtsc_mig_blocker,
> + "State blocked by non-migratable CPU device"
> + " (invtsc flag)");
> + migrate_add_blocker(invtsc_mig_blocker);
> + /* for savevm */
> + vmstate_x86_cpu.unmigratable = 1;
> }
>
> cpuid_data.cpuid.padding = 0;
> --
> 2.7.4