Re: [Qemu-devel] [PATCH] Use Aff1 with mpidr

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 29 May 2015 11:53:38 +0300
Pavel Fedin <p.fedin@samsung.com> wrote:

>  Hello!
> 
> > > +    /*
> > > +     * When KVM is in use, psci is emulated in-kernel and not by qemu.
> > > +     * In order for it to work correctly we should use correct MPIDR values,
> > > +     * which appear to be inherited from the host.
> > why it must be inherited from host?
> > Could you point out to KVM code that depends on it?
> 
>  It is PSCI, i have tested this and experienced this problem. In KVM guest PSCI is handled
> by "hvc" call and is done entirely in kernel, no qemu code is involved. If i supply wrong
> IDs the processors will simply not power up.
>  I have checked how kvmtool (found here: git://linux-arm.org/linux-ap.git, tools/kvm)
> handles this. Related parts are:
> tools/kvm/arm/fdt.c - generate_cpu_nodes() - "reg" property assignment:
> --- cut ---
> static void generate_cpu_nodes(void *fdt, struct kvm *kvm)
> {
> 	int cpu;
> 
> 	_FDT(fdt_begin_node(fdt, "cpus"));
> 	_FDT(fdt_property_cell(fdt, "#address-cells", 0x1));
> 	_FDT(fdt_property_cell(fdt, "#size-cells", 0x0));
> 
> 	for (cpu = 0; cpu < kvm->nrcpus; ++cpu) {
> 		char cpu_name[CPU_NAME_MAX_LEN];
> 		struct kvm_cpu *vcpu = kvm->cpus[cpu];
> 		unsigned long mpidr = kvm_cpu__get_vcpu_mpidr(vcpu);
> 
> 		mpidr &= ARM_MPIDR_HWID_BITMASK;
> 		snprintf(cpu_name, CPU_NAME_MAX_LEN, "cpu@%lx", mpidr);
> 
> 		_FDT(fdt_begin_node(fdt, cpu_name));
> 		_FDT(fdt_property_string(fdt, "device_type", "cpu"));
> 		_FDT(fdt_property_string(fdt, "compatible", vcpu->cpu_compatible));
> 
> 		if (kvm->nrcpus > 1)
> 			_FDT(fdt_property_string(fdt, "enable-method", "psci"));
> 
> 		_FDT(fdt_property_cell(fdt, "reg", mpidr));
> 		_FDT(fdt_end_node(fdt));
> 	}
> 
> 	_FDT(fdt_end_node(fdt));
> }
> --- cut ---
>  tools/kvm/arm/aarch64/kvm-cpu.c - kvm_cpu__get_vcpu_mpidr() - obtains CPU ID from the
> kernel:
> --- cut ---
> unsigned long kvm_cpu__get_vcpu_mpidr(struct kvm_cpu *vcpu)
> {
> 	struct kvm_one_reg reg;
> 	u64 mpidr;
> 
> 	reg.id = ARM64_SYS_REG(ARM_CPU_ID, ARM_CPU_ID_MPIDR);
> 	reg.addr = (u64)&mpidr;
> 	if (ioctl(vcpu->vcpu_fd, KVM_GET_ONE_REG, &reg) < 0)
> 		die("KVM_GET_ONE_REG failed (get_mpidr vcpu%ld", vcpu->cpu_id);
> 
> 	return mpidr;
> }
> --- cut ---
> 
>  So i just decided to do the same thing in KVM. However, to tell the truth, i actually do
> not know whether it is possible to do it in reverse and assign MPIDR to VCPUs using
> KVM_SET_ONE_REG ioctl instead. I can try it if you think it's more appropriate.
That's what I see as correct way to go
and along the way teach KVM to not derive mpidr encoding from vcpuid
and use QEMU provided mpidr value, that way mpidr would be consistent.

As it currently stands QEMU notion of mpidr and KVM's will diverge
onece CPU count goes above 16 CPUs.

>  
> Kind regards,
> Pavel Fedin
> Expert Engineer
> Samsung Electronics Research center Russia
> 
>