Re: [Qemu-devel] [PATCH qemu] spapr/target-ppc/kvm: Only add hcall-instructions if KVM supports it

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 03/15/2016 10:32 PM, Thomas Huth wrote:
> On 15.03.2016 10:42, Alexey Kardashevskiy wrote:
>> On 03/15/2016 07:18 PM, Thomas Huth wrote:
>>>
>>>    Hi Alexey,
>>>
>>> On 15.03.2016 06:51, Alexey Kardashevskiy wrote:
>>>> ePAPR defines "hcall-instructions" device-tree property which contains
>>>> code to call hypercalls in ePAPR paravirtualized guests. However this
>>>> property is also present for pseries guests where it does not make
>>>> sense,
>>>> even though it contains dummy code which simply fails.
>>>>
>>>> Instead of maintaining the property (which used to be BE only; then was
>>>> fixed to be endian-agnostic) and confusing the guest (which might think
>>>> there is ePAPR host while there is none), this simply does not
>>>> the property to the device tree if the host kernel does not implement
>>>> it.
>>>>
>>>> In order to tell the machine code if the host kernel supports
>>>> KVM_CAP_PPC_GET_PVINFO, this changes kvmppc_get_hypercall() to return 1
>>>> if the host kernel does not implement it (which is HV KVM case).
>>>>
>>>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>>>> ---
>>>>
>>>>
>>>> Alexander,
>>>>
>>>> We just got a bug report that LE guests would not boot under quite
>>>> old QEMU
>>>> and we (powerkvm) wonder if it makes sense to backport endian-agnostic
>>>> hypercall code to older QEMU or it is simpler/more correct
>>>> not to have epapr-hypercall property in the tree.
>>>>
>>>>
>>>> ---
>>>>    hw/ppc/spapr.c   | 9 +++++----
>>>>    target-ppc/kvm.c | 2 +-
>>>>    2 files changed, 6 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
>>>> index 43708a2..8130eb4 100644
>>>> --- a/hw/ppc/spapr.c
>>>> +++ b/hw/ppc/spapr.c
>>>> @@ -497,10 +497,11 @@ static void *spapr_create_fdt_skel(hwaddr
>>>> initrd_base,
>>>>                 * Older KVM versions with older guest kernels were
>>>> broken with the
>>>>                 * magic page, don't allow the guest to map it.
>>>>                 */
>>>> -            kvmppc_get_hypercall(first_cpu->env_ptr, hypercall,
>>>> -                                 sizeof(hypercall));
>>>> -            _FDT((fdt_property(fdt, "hcall-instructions", hypercall,
>>>> -                              sizeof(hypercall))));
>>>> +            if (!kvmppc_get_hypercall(first_cpu->env_ptr, hypercall,
>>>> +                                      sizeof(hypercall))) {
>>>> +                _FDT((fdt_property(fdt, "hcall-instructions",
>>>> hypercall,
>>>> +                                   sizeof(hypercall))));
>>>> +            }
>>>>            }
>>>>            _FDT((fdt_end_node(fdt)));
>>>>        }
>>>> diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
>>>> index 776336b..e5183db 100644
>>>> --- a/target-ppc/kvm.c
>>>> +++ b/target-ppc/kvm.c
>>>> @@ -2001,7 +2001,7 @@ int kvmppc_get_hypercall(CPUPPCState *env,
>>>> uint8_t *buf, int buf_len)
>>>>        hc[2] = cpu_to_be32(0x48000008);
>>>>        hc[3] = cpu_to_be32(bswap32(0x3860ffff));
>>>>
>>>> -    return 0;
>>>> +    return 1;
>>>>    }
>>>>
>>>>    static inline int kvmppc_enable_hcall(KVMState *s, target_ulong hcall)
>>>
>>> Sorry, I have a hard time to understand what this is really good for. Is
>>> it a patch for current QEMU or for older ones? If it is for older ones,
>>> then why did you not CC: to qemu-stable?
>>> If it is for current QEMU, then I've got some more questions about
>>> things I do not understand:
>>>
>>> 1) In your patch description, you talk about ePAPR and that the property
>>> does not make sense for pseries. But why is this code then available at
>>> all in spapr.c? ... there must be a reason for this, I think (like using
>>> a different h-call on nested KVM-PR for example?)
>>
>>
>> No, this is from old times when there was only PR KVM fully emulating
>> powermac (not pseries) which needed to interact with the hypervisor and
>> epapr_hypercall was chosen for this.
>>
>>
>>> 2) The code in spapr.c is already protected with a
>>>     if (kvmppc_has_cap_fixup_hcalls()) ...
>>> and that CAP should only be there if the PVINFO CAP is available, too.
>>> So I don't see how you could run into that problem anyway where PVINFO
>>> is _not_ available but the FIXUP_HCALL CAP _is_ available?
>>
>>
>> HV KVM guest calls (on pseries machine as well):
>>
>> kvm_guest_init
>> kvm_para_has_feature
>> kvm_arch_para_features
>> kvm_para_available - this returns "1"
>> epapr_hypercall0_1(KVM_HC_FEATURES)
>>
>> This epapr_hypercall0_1() calls a binary blob from "hcall-instructions".
>> And fails if the guest is LE and the blob from BE-only times.
>
> What about that "if (kvmppc_has_cap_fixup_hcalls())" ? Could you please
> check why this succeeds on your system , but the KVM_CAP_PPC_GET_PVINFO
> call does not?

KVM_CAP_PPC_FIXUP_HCALL is always enabled for CONFIG_PPC_BOOK3S_64, 
KVM_CAP_PPC_GET_PVINFO is only enabled for "!hv_enabled".



-- 
Alexey