Re: [PATCH v5 3/3] Add support for RAPL MSRs in KVM/Qemu

[Anthony Harivel]

Hi Paolo,

Daniel P. Berrangé, Apr 25, 2024 at 17:42:
> On Thu, Apr 25, 2024 at 05:34:52PM +0200, Anthony Harivel wrote:
> > Hi Daniel,
> > 
> > Daniel P. Berrangé, Apr 18, 2024 at 18:42:
> > 
> > > > +    if (kvm_is_rapl_feat_enable(cs)) {
> > > > +        if (!IS_INTEL_CPU(env)) {
> > > > +            error_setg(errp, "RAPL feature can only be\
> > > > +                              enabled with Intel CPU models");
> > > > +                return false;
> > > > +        }
> > > > +    }
> > >
> > > I see a crash in kvm_is_rapl_feat_enable() from this caller,
> > > when I run with this kind of command line:
> > >
> > >  $ qemu-system-x86_64 \
> > >       -kernel /lib/modules/6.6.9-100.fc38.x86_64/vmlinuz \
> > >       -initrd tiny-initrd.img  -m 2000 -serial stdio -nodefaults \
> > >       -display none -accel kvm -append "console=ttyS0 quiet"
> > >
> > >
> > > #0  0x0000555555bc14b7 in kvm_is_rapl_feat_enable 
> > > (cs=cs@entry=0x555557b83470) at ../target/i386/kvm/kvm.c:2531
> > > #1  0x0000555555bc7534 in kvm_cpu_realizefn (cs=0x555557b83470, 
> > > errp=0x7fffffffd2a0) at ../target/i386/kvm/kvm-cpu.c:54
> > > #2  0x0000555555d2432a in accel_cpu_common_realize (cpu=0x555557b83470, 
> > > errp=0x7fffffffd2a0) at ../accel/accel-target.c:130
> > > #3  0x0000555555cdd955 in cpu_exec_realizefn 
> > > (cpu=cpu@entry=0x555557b83470, errp=errp@entry=0x7fffffffd2a0) at 
> > > ../cpu-target.c:137
> > > #4  0x0000555555c14b89 in x86_cpu_realizefn (dev=0x555557b83470, 
> > > errp=0x7fffffffd310) at ../target/i386/cpu.c:7320
> > > #5  0x0000555555d58f4b in device_set_realized (obj=<optimized out>, 
> > > value=<optimized out>, errp=0x7fffffffd390) at ../hw/core/qdev.c:510
> > > #6  0x0000555555d5d78d in property_set_bool (obj=0x555557b83470, 
> > > v=<optimized out>, name=<optimized out>, opaque=0x5555578558e0, 
> > > errp=0x7fffffffd390)
> > >     at ../qom/object.c:2358
> > > #7  0x0000555555d60b0b in object_property_set 
> > > (obj=obj@entry=0x555557b83470, name=name@entry=0x55555607c799 "realized", 
> > > v=v@entry=0x555557b8ccb0, errp=0x7fffffffd390, 
> > >     errp@entry=0x555556e210d8 <error_fatal>) at ../qom/object.c:1472
> > > #8  0x0000555555d6444f in object_property_set_qobject
> > >     (obj=obj@entry=0x555557b83470, name=name@entry=0x55555607c799 
> > > "realized", value=value@entry=0x555557854800, 
> > > errp=errp@entry=0x555556e210d8 <error_fatal>)
> > >     at ../qom/qom-qobject.c:28
> > > #9  0x0000555555d61174 in object_property_set_bool
> > >     (obj=0x555557b83470, name=name@entry=0x55555607c799 "realized", 
> > > value=value@entry=true, errp=errp@entry=0x555556e210d8 <error_fatal>) at 
> > > ../qom/object.c:1541
> > > #10 0x0000555555d59a3c in qdev_realize (dev=<optimized out>, 
> > > bus=bus@entry=0x0, errp=errp@entry=0x555556e210d8 <error_fatal>) at 
> > > ../hw/core/qdev.c:292
> > > #11 0x0000555555bd51e0 in x86_cpu_new (x86ms=<optimized out>, apic_id=0, 
> > > errp=0x555556e210d8 <error_fatal>) at ../hw/i386/x86.c:105
> > > #12 0x0000555555bd52ce in x86_cpus_init 
> > > (x86ms=x86ms@entry=0x555557aaed30, default_cpu_version=<optimized out>) 
> > > at ../hw/i386/x86.c:156
> > > #13 0x0000555555bdc1a7 in pc_init1 (machine=0x555557aaed30, 
> > > pci_type=0x55555604aa61 "i440FX") at ../hw/i386/pc_piix.c:185
> > > #14 0x0000555555947a11 in machine_run_board_init (machine=0x555557aaed30, 
> > > mem_path=<optimized out>, errp=<optimized out>, errp@entry=0x555556e210d8 
> > > <error_fatal>)
> > >     at ../hw/core/machine.c:1547
> > > #15 0x0000555555b020ed in qemu_init_board () at ../system/vl.c:2613
> > > #16 qmp_x_exit_preconfig (errp=0x555556e210d8 <error_fatal>) at 
> > > ../system/vl.c:2705
> > > #17 0x0000555555b0611e in qemu_init (argc=<optimized out>, 
> > > argv=<optimized out>) at ../system/vl.c:3739
> > > #18 0x0000555555897ca9 in main (argc=<optimized out>, argv=<optimized 
> > > out>) at ../system/main.c:47
> > >
> > > The problem is that 'cs->kvm_state' is NULL here
> > >
> > 
> > After some investigation it seems that kvm_state is not yet committed 
> > at this point. Shame, because GDB showed me that we have already pass 
> > the kvm_accel_instance_init() in accel/kvm/kvm-all.c that sets the 
> > value "msr_energy.enable" in kvm_state...
> > 
> > So should I dig more to still do the sanity check in kvm_cpu_realizefn() 
> > or should I already move the RAPL feature  from -kvm to -cpu 
> > like suggested by Zhao from Intel and then access it from the CPUState ?
>
> I'm not so sure about that question. I think Paolo is best placed
> to suggest which is better, as the KVM maintainer.
>

I'm facing an issue that either require a simple change or a more 
complex one depending on the decision.

TL;DR: Should I move from -kvm to -cpu the rapl feature ? 

Zhao from Intel suggested me that enabling the rapl feature looks more 
natural than through -kvm feature because it is indeed a cpu feature. 

>From the point of view of the QEMU architecture, it's more easier to 
enable the feature with -kvm because it is kvm dependent; but maybe from 
the point of view of the user, it's more natural to enable this at -cpu 
level. 

The issue I'm facing above is from a suggestion from Daniel to do the 
sanity check at kvm_cpu_realizefn() level. However GDB showed me that 
kvm_state is not yet populated, even if we have passed the init instance 
kvm_accel_instance_init().

So either I'm moving from -kvm to -cpu and change the location of the 
sanity check (checking if rapl feat is activated on Host / if Intel CPU) 
or I keep the feature on -kvm and either find a solution of the above 
issue or change the location of the sanity check.

Thanks in advance for your guidance.

Regards,
Anthony