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

[David Gibson]

On Tue, May 05, 2015 at 08:42:36AM +0200, Thomas Huth wrote:
> On Tue,  5 May 2015 11:00:01 +1000
> David Gibson <address@hidden> 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.

I'm not sure if there's such a function, but even with it, I can't see
a way to do this that isn't really fragile.  virtio-blk is only
affected if using iothread / dataplane - not if fully handled in
qemu.  There may be other devices which will be affected if using
dataplane for the same reasons, and if there aren't now there may well
be in future.

Likewise any future devices which could have a KVM implementation
would be affected, and there's no obvious way to enumerate what those
are.

> 
> > Signed-off-by: David Gibson <address@hidden>
> > ---
> >  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 <address@hidden>
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

pgpcOLzMHvDQp.pgp
Description: PGP signature