RE: [PATCH v13 1/5] i386: Add support for CPUID_8000_001E for AMD

[Babu Moger <babu.moger@amd.com>]

> -----Original Message-----
> From: Eduardo Habkost <ehabkost@redhat.com>
> Sent: Tuesday, June 2, 2020 12:52 PM
> To: Moger, Babu <Babu.Moger@amd.com>
> Cc: mst@redhat.com; marcel.apfelbaum@gmail.com; pbonzini@redhat.com;
> rth@twiddle.net; mtosatti@redhat.com; qemu-devel@nongnu.org;
> kvm@vger.kernel.org; kash@tripleback.net; geoff@hostfission.com; Dr. David
> Alan Gilbert <dgilbert@redhat.com>
> Subject: Re: [PATCH v13 1/5] i386: Add support for CPUID_8000_001E for AMD
> 
> On Fri, Jun 08, 2018 at 06:56:17PM -0400, Babu Moger wrote:
> > Add support for cpuid leaf CPUID_8000_001E. Build the config that closely
> > match the underlying hardware. Please refer to the Processor Programming
> > Reference (PPR) for AMD Family 17h Model for more details.
> >
> > Signed-off-by: Babu Moger <babu.moger@amd.com>
> [...]
> > +    case 0x8000001E:
> > +        assert(cpu->core_id <= 255);
> 
> It is possible to trigger this assert using:
> 
> $ qemu-system-x86_64 -machine q35,accel=kvm,kernel-irqchip=split -device
> intel-iommu,intremap=on,eim=on -smp
> 1,maxcpus=258,cores=258,threads=1,sockets=1 -cpu
> qemu64,xlevel=0x8000001e -device qemu64-x86_64-cpu,apic-id=257
> qemu-system-x86_64: warning: Number of hotpluggable cpus requested (258)
> exceeds the recommended cpus supported by KVM (240)
> qemu-system-x86_64:
> /home/ehabkost/rh/proj/virt/qemu/target/i386/cpu.c:5888: cpu_x86_cpuid:
> Assertion `cpu->core_id <= 255' failed.
> Aborted (core dumped)
> 
> See bug report and discussion at
> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.
> redhat.com%2Fshow_bug.cgi%3Fid%3D1834200&amp;data=02%7C01%7Cbabu.
> moger%40amd.com%7C8a2724729b914bc9b53d08d8071db392%7C3dd8961fe4
> 884e608e11a82d994e183d%7C0%7C0%7C637267171438806408&amp;sdata=ib
> iGlF%2FF%2FVtYQLf7fe988kxFsLhj4GrRiTOq4LUuOT8%3D&amp;reserved=0
> 
> Also, it looks like encode_topo_cpuid8000001e() assumes core_id
> has only 3 bits, so the existing assert() is not even sufficient.
> We need to decide what to do if the user requests nr_cores > 8.
> 
> Probably omitting CPUID[0x8000001E] if the VCPU topology is
> incompatible with encode_topo_cpuid8000001e() (and printing a
> warning) is the safest thing to do right now.

Eduardo,  We need to generalize the encode_topo_cpuid8000001e decoding.
We will have to remove 3 bit limitation there. It will not scale with
latest configurations. I will take a look that.

For now, best option I think is to(like you mentioned in bug 1834200),
declaring nr_cores > 256 as never supported (or deprecated); and throw
warning.

What do you think?
> 
> 
> 
> > +        encode_topo_cpuid8000001e(cs, cpu,
> > +                                  eax, ebx, ecx, edx);
> > +        break;
> >      case 0xC0000000:
> >          *eax = env->cpuid_xlevel2;
> >          *ebx = 0;
> > --
> > 1.8.3.1
> >
> 
> --
> Eduardo