Re: [Qemu-devel] [PATCH] pseries: Enable in-kernel H_LOGICAL_CI_{LOAD, STORE} implementations

[Thomas Huth <thuth@redhat.com>]

On Tue,  5 May 2015 11:00:01 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> qemu currently implements the hypercalls H_LOGICAL_CI_LOAD and
> H_LOGICAL_CI_STORE as PAPR extensions.  These are used by the SLOF firmware
> for IO, because performing cache inhibited MMIO accesses with the MMU off
> (real mode) is very awkward on POWER.
> 
> This approach breaks when SLOF needs to access IO devices implemented
> within KVM instead of in qemu.  The simplest example would be virtio-blk
> using an iothread, because the iothread / dataplane mechanism relies on
> an in-kernel implementation of the virtio queue notification MMIO.
> 
> To fix this, an in-kernel implementation of these hypercalls has been made,
> (kernel commit 99342cf "kvmppc: Implement H_LOGICAL_CI_{LOAD,STORE} in KVM"
> however, the hypercalls still need to be enabled from qemu.  This performs
> the necessary calls to do so.
> 
> It would be nice to provide some warning if we encounter a problematic
> device with a kernel which doesn't support the new calls.  Unfortunately,
> I can't see a way to detect this case which won't either warn in far too
> many cases that will probably work, or which is horribly invasive.

Hmm, is there a function that returns you a list to a given type?
Something like object_resolve_path(TYPE_VIRTIO_BLK, NULL) but not only
returning one matching object but all? ... then you could step through
all the possibly affected devices and check whether they need this
kernel feature.

> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  hw/ppc/spapr.c       |  5 +++++
>  target-ppc/kvm.c     | 18 ++++++++++++++++++
>  target-ppc/kvm_ppc.h |  5 +++++
>  3 files changed, 28 insertions(+)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 644689a..3b5768b 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -1504,6 +1504,11 @@ static void ppc_spapr_init(MachineState *machine)
>          qemu_register_reset(spapr_cpu_reset, cpu);
>      }
>  
> +    if (kvm_enabled()) {
> +        /* Enable H_LOGICAL_CI_* so SLOF can talk to in-kernel devices */
> +        kvmppc_enable_logical_ci_hcalls();
> +    }
> +
>      /* allocate RAM */
>      spapr->ram_limit = ram_size;
>      memory_region_allocate_system_memory(ram, NULL, "ppc_spapr.ram",
> diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
> index 12328a4..fde26d0 100644
> --- a/target-ppc/kvm.c
> +++ b/target-ppc/kvm.c
> @@ -1882,6 +1882,24 @@ int kvmppc_get_hypercall(CPUPPCState *env, uint8_t *buf, int buf_len)
>      return 0;
>  }
>  
> +static inline int kvmppc_enable_hcall(KVMState *s, target_ulong hcall)
> +{
> +    return kvm_vm_enable_cap(s, KVM_CAP_PPC_ENABLE_HCALL, 0, hcall, 1);
> +}
> +
> +void kvmppc_enable_logical_ci_hcalls(void)
> +{
> +    /*
> +     * FIXME: it would be nice if we could detect the cases where
> +     * we're using a device which requires the in kernel
> +     * implementation of these hcalls, but the kernel lacks them and
> +     * produce a warning.  So far I haven't seen a practical way to do
> +     * that
> +     */

I'd maybe drop or shorten this comment (at least the last sentence).

> +    kvmppc_enable_hcall(kvm_state, H_LOGICAL_CI_LOAD);
> +    kvmppc_enable_hcall(kvm_state, H_LOGICAL_CI_STORE);
> +}
> +
>  void kvmppc_set_papr(PowerPCCPU *cpu)
>  {
>      CPUState *cs = CPU(cpu);
> diff --git a/target-ppc/kvm_ppc.h b/target-ppc/kvm_ppc.h
> index 2e0224c..4d30e27 100644
> --- a/target-ppc/kvm_ppc.h
> +++ b/target-ppc/kvm_ppc.h
> @@ -24,6 +24,7 @@ bool kvmppc_get_host_serial(char **buf);
>  int kvmppc_get_hasidle(CPUPPCState *env);
>  int kvmppc_get_hypercall(CPUPPCState *env, uint8_t *buf, int buf_len);
>  int kvmppc_set_interrupt(PowerPCCPU *cpu, int irq, int level);
> +void kvmppc_enable_logical_ci_hcalls(void);
>  void kvmppc_set_papr(PowerPCCPU *cpu);
>  int kvmppc_set_compat(PowerPCCPU *cpu, uint32_t cpu_version);
>  void kvmppc_set_mpic_proxy(PowerPCCPU *cpu, int mpic_proxy);
> @@ -107,6 +108,10 @@ static inline int kvmppc_set_interrupt(PowerPCCPU *cpu, int irq, int level)
>      return -1;
>  }
>  
> +static inline void kvmppc_enable_logical_ci_hcalls(void)
> +{
> +}
> +
>  static inline void kvmppc_set_papr(PowerPCCPU *cpu)
>  {
>  }

Reviewed-by: Thomas Huth <thuth@redhat.com>