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

[Daniel P. Berrange]

On Mon, Apr 24, 2017 at 10:23:21AM +0100, Dr. David Alan Gilbert wrote:
> * Zhuangyanying (address@hidden) 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 ?

Regards,
Daniel
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