Re: [Qemu-devel] [BUG] Migrate failes between boards with different PMC counts

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Apr 24, 2017 at 11:27:16AM +0100, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrange (berrange@redhat.com) wrote:
> > On Mon, Apr 24, 2017 at 10:23:21AM +0100, Dr. David Alan Gilbert wrote:
> > > * Zhuangyanying (ann.zhuangyanying@huawei.com) wrote:
> > > > Hi all,
> > > > 
> > > > Recently, I found migration failed when enable vPMU.
> > > > 
> > > > migrate vPMU state was introduced in linux-3.10 + qemu-1.7.
> > > > 
> > > > As long as enable vPMU, qemu will save / load the
> > > > vmstate_msr_architectural_pmu(msr_global_ctrl) register during the migration.
> > > > But global_ctrl generated based on cpuid(0xA), the number of general-purpose performance
> > > > monitoring counters(PMC) can vary according to Intel SDN. The number of PMC presented
> > > > to vm, does not support configuration currently, it depend on host cpuid, and enable all pmc
> > > > defaultly at KVM. It cause migration to fail between boards with different PMC counts.
> > > > 
> > > > The return value of cpuid (0xA) is different dur to cpu, according to Intel SDN，18-10 Vol. 3B:
> > > > 
> > > > Note: The number of general-purpose performance monitoring counters (i.e. N in Figure 18-9)
> > > > can vary across processor generations within a processor family, across processor families, or
> > > > could be different depending on the configuration chosen at boot time in the BIOS regarding
> > > > Intel Hyper Threading Technology, (e.g. N=2 for 45 nm Intel Atom processors; N =4 for processors
> > > > based on the Nehalem microarchitecture; for processors based on the Sandy Bridge
> > > > microarchitecture, N = 4 if Intel Hyper Threading Technology is active and N=8 if not active).
> > > > 
> > > > Also I found, N=8 if HT is not active based on the broadwell，,
> > > > such as CPU E7-8890 v4 @ 2.20GHz   
> > > > 
> > > > # ./x86_64-softmmu/qemu-system-x86_64 --enable-kvm -smp 4 -m 4096 -hda
> > > > /data/zyy/test_qemu.img.sles12sp1 -vnc :99 -cpu kvm64,pmu=true -incoming tcp::8888
> > > > Completed 100 %
> > > > qemu-system-x86_64: error: failed to set MSR 0x38f to 0x7000000ff
> > > > qemu-system-x86_64: /data/zyy/git/test/qemu/target/i386/kvm.c:1833: kvm_put_msrs: 
> > > > Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.
> > > > Aborted
> > > > 
> > > > So make number of pmc configurable to vm ? Any better idea ?
> > > 
> > > Coincidentally we hit a similar problem a few days ago with -cpu host  - it took me
> > > quite a while to spot the difference between the machines was the source
> > > had hyperthreading disabled.
> > > 
> > > An option to set the number of counters makes sense to me; but I wonder
> > > how many other options we need as well.  Also, I'm not sure there's any
> > > easy way for libvirt etc to figure out how many counters a host supports -
> > > it's not in /proc/cpuinfo.
> > 
> > We actually try to avoid /proc/cpuinfo whereever possible. We do direct
> > CPUID asm instructions to identify features, and prefer to use
> > /sys/devices/system/cpu if that has suitable data
> > 
> > Where do the PMC counts come from originally ? CPUID or something else ?
> 
> Yes, they're bits 8..15 of CPUID leaf 0xa

Ok, that's easy enough for libvirt to detect then. More a question of what
libvirt should then do this with the info....

Regards,
Daniel
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