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

[Thomas Huth <thuth@redhat.com>]

 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?)

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?

 Thomas