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 08:59 PM, David Gibson wrote:
> On Tue, Mar 15, 2016 at 04:51:20PM +1100, 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>
>
> So the idea of only adding the property when the host kernel supplies
> a suitable value seems good, but I'm a bit nervous about applying
> this, because I'm not sure what case the original fallback hypercall
> code was supposed to handle.
>
> agraf, if you could enlighten us with some history that could be good.
>
>> 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));
>
> Since you're now returning a value which means the caller is supposed
> to ignore the hc code, there's not much point actually populating it above.


The return value means "no KVM support is here" rather than "ignore @buf 
content".

And the patch should have been "RFC" I suppose :)

>
>> -    return 0;
>> +    return 1;
>>   }
>>
>>   static inline int kvmppc_enable_hcall(KVMState *s, target_ulong hcall)
>


-- 
Alexey