On Tue, Sep 08, 2020 at 07:25:47PM +0200, Thomas Huth wrote:
> On 24/08/2020 10.11, Huacai Chen wrote:
> > MIPS has two types of KVM: TE & VZ, and TE is the default type. Now,
> > libvirt uses a null-machine to detect the kvm capability. In the MIPS
> > case, it will return "KVM not supported" on a VZ platform by default.
> > So, add the kvm_type() hook to the null-machine.
> >
> > This seems not a very good solution, but I cannot do it better now.
>
> This is still ugly. Why do the other architectures do not have the
> same problem? Let's see... in kvm-all.c, we have:
>
> int type = 0;
> [...]
> kvm_type = qemu_opt_get(qemu_get_machine_opts(), "kvm-type");
> if (mc->kvm_type) {
> type = mc->kvm_type(ms, kvm_type);
> } else if (kvm_type) {
> ret = -EINVAL;
> fprintf(stderr, "Invalid argument kvm-type=%s\n", kvm_type);
> goto err;
> }
>
> do {
> ret = kvm_ioctl(s, KVM_CREATE_VM, type);
> } while (ret == -EINTR);
>
> Thus the KVM_CREATE_VM ioctl is likely called with type = 0 in this
> case (i.e. when libvirt probes with the "null"-machine).
>
> Now let's have a look at the kernel. The "type" parameter is passed
> there to the architecture specific function kvm_arch_init_vm().
> For powerpc, this looks like:
>
> if (type == 0) {
> if (kvmppc_hv_ops)
> kvm_ops = kvmppc_hv_ops;
> else
> kvm_ops = kvmppc_pr_ops;
> if (!kvm_ops)
> goto err_out;
> } else if (type == KVM_VM_PPC_HV) {
> if (!kvmppc_hv_ops)
> goto err_out;
> kvm_ops = kvmppc_hv_ops;
> } else if (type == KVM_VM_PPC_PR) {
> if (!kvmppc_pr_ops)
> goto err_out;
> kvm_ops = kvmppc_pr_ops;
> } else
> goto err_out;
>
> That means for type == 0, it automatically detects the best
> kvm-type.
>
> For mips, this function looks like this:
>
> switch (type) {
> #ifdef CONFIG_KVM_MIPS_VZ
> case KVM_VM_MIPS_VZ:
> #else
> case KVM_VM_MIPS_TE:
> #endif
> break;
> default:
> /* Unsupported KVM type */
> return -EINVAL;
> };
>
> That means, for type == 0, it returns -EINVAL here!
>
> Looking at the API docu in Documentation/virt/kvm/api.rst
> the description of the type parameter is quite sparse, but it
> says:
>
> "You probably want to use 0 as machine type."
>
> So I think this is a bug in the implementation of KVM in the
> mips kernel code. The kvm_arch_init_vm() in the mips code should
> do the same as on powerpc, and use the best available KVM type
> there instead of returning EINVAL. Once that is fixed there,
> you don't need this patch here for QEMU anymore.
If there's a way to make it work with older kernels, I assume we
would still want to do it.
However, this kind of kvm-specific + arch-specific knowledge
doesn't belong to machine core code. If we are going to add a
#ifdef TARGET_MIPS to the code, we can simply do it inside
kvm-all.c:kvm_init().
--
Eduardo
