Re: [PATCH V2 for-5.2] hw/null-machine: Add the kvm_type() hook for MIPS

[Thomas Huth <thuth@redhat.com>]

On 09/09/2020 04.57, chen huacai wrote:
> Hi, all,
> 
> On Wed, Sep 9, 2020 at 1:25 AM Thomas Huth <thuth@redhat.com> 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.
> Yes, PPC use a good method, because it can use 0 as "automatic"
> #define KVM_VM_PPC_HV 1
> #define KVM_VM_PPC_PR 2
> 
> Unfortunately, MIPS cannot do like this because it define 0 as "TE":
> #define KVM_VM_MIPS_TE          0
> #define KVM_VM_MIPS_VZ          1
> 
> So, it cannot be solved in kernel side, unless changing the definition
> of TE/VZ, but I think changing their definition is also unacceptable.

Ouch, ok, now I understood the problem. That sounds like a really bad
decision on the kernel side.

But I think you could at least try to get it fixed on the kernel side:
Propose a patch to rename KVM_VM_MIPS_TE to KVM_VM_MIPS_DEFAULT and use
2 as new value for TE. The code that handles the default 0 case should
then prefer TE over VZ, so that old userspace applications that provide
0 will continue to work as they did before, so I hope that the change is
acceptable by the kernel folks. You should add a remark to the patch
description that 0 is the established default value for probing in the
QEMU/libvirt stack and that your patch is required on the kernel side to
avoid ugly hacks in QEMU userspace code.

If they still reject your patch, I guess we have to bite the bullet and
add your patch here...

Thanks,
 Thomas