Re: [Qemu-ppc] [RFC for-2.12 7/8] spapr: Handle VMX/VSX presence as an spapr capability flag

[Greg Kurz]

On Mon, 11 Dec 2017 18:08:07 +1100
David Gibson <address@hidden> wrote:

> We currently have some conditionals in the spapr device tree code to decide
> whether or not to advertise the availability of the VMX (aka Altivec) and
> VSX vector extensions to the guest, based on whether the guest cpu has
> those features.
> 
> This can lead to confusion and subtle failures on migration, since it makes
> a guest visible change based only on host capabilities.  We now have a
> better mechanism for this, in spapr capabilities flags, which explicitly
> depend on user options rather than host capabilities.
> 
> Rework the advertisement of VSX and VMX based on a new VSX capability.  We
> no longer bother with a conditional for VMX support, because every CPU
> that's ever been supported by the pseries machine type supports VMX.
> 
> NOTE: Some userspace distributions (e.g. RHEL7.4) already rely on
> availability of VSX in libc, so using cap-vsx=off may lead to a fatal
> SIGILL in init.
> 
> Signed-off-by: David Gibson <address@hidden>
> ---

Just one nit, see below, but anyway:

Reviewed-by: Greg Kurz <address@hidden>


>  hw/ppc/spapr.c         | 18 ++++++++++--------
>  hw/ppc/spapr_caps.c    | 24 ++++++++++++++++++++++++
>  include/hw/ppc/spapr.h |  3 +++
>  3 files changed, 37 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index e206825ed9..6b37511e8f 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -556,14 +556,16 @@ static void spapr_populate_cpu_dt(CPUState *cs, void 
> *fdt, int offset,
>                            segs, sizeof(segs))));
>      }
>  
> -    /* Advertise VMX/VSX (vector extensions) if available
> -     *   0 / no property == no vector extensions
> +    /* Advertise VSX (vector extensions) if available
>       *   1               == VMX / Altivec available
> -     *   2               == VSX available */
> -    if (env->insns_flags & PPC_ALTIVEC) {
> -        uint32_t vmx = (env->insns_flags2 & PPC2_VSX) ? 2 : 1;
> -
> -        _FDT((fdt_setprop_cell(fdt, offset, "ibm,vmx", vmx)));
> +     *   2               == VSX available
> +     *
> +     * Only CPUs for which we create core types in spapr_cpu_core.c
> +     * are possible, and all of those have VMX */
> +    if (spapr_has_cap(spapr, SPAPR_CAP_VSX)) {
> +        _FDT((fdt_setprop_cell(fdt, offset, "ibm,vmx", 2)));

Maybe a good opportunity to drop the extra parens since the _FDT() macro
already adds them around its argument.

#define _FDT(exp)                                                  \
    do {                                                           \
        int ret = (exp);                                           \


> +    } else {
> +        _FDT((fdt_setprop_cell(fdt, offset, "ibm,vmx", 1)));
>      }
>  
>      /* Advertise DFP (Decimal Floating Point) if available
> @@ -3829,7 +3831,7 @@ static void spapr_machine_class_init(ObjectClass *oc, 
> void *data)
>       */
>      mc->numa_mem_align_shift = 28;
>  
> -    smc->default_caps = spapr_caps(SPAPR_CAP_HTM);
> +    smc->default_caps = spapr_caps(SPAPR_CAP_HTM | SPAPR_CAP_VSX);
>      spapr_capability_properties(smc, &error_abort);
>  }
>  
> diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
> index 6bba04d60e..b19a9f4417 100644
> --- a/hw/ppc/spapr_caps.c
> +++ b/hw/ppc/spapr_caps.c
> @@ -44,6 +44,11 @@ static sPAPRCapabilityInfo capability_table[] = {
>          .description = "Allow Hardware Transactional Memory (HTM)",
>          .bit = SPAPR_CAP_HTM,
>      },
> +    {
> +        .name = "vsx",
> +        .description = "Allow Vector Scalar Extensions (VSX)",
> +        .bit = SPAPR_CAP_VSX,
> +    },
>  };
>  
>  static sPAPRCapabilities default_caps_with_cpu(sPAPRMachineState *spapr, 
> CPUState *cs)
> @@ -59,6 +64,11 @@ static sPAPRCapabilities 
> default_caps_with_cpu(sPAPRMachineState *spapr, CPUStat
>          caps.mask &= ~SPAPR_CAP_HTM;
>      }
>  
> +    if (!ppc_check_compat(cpu, CPU_POWERPC_LOGICAL_2_06,
> +                          0, spapr->max_compat_pvr)) {
> +        caps.mask &= ~SPAPR_CAP_VSX;
> +    }
> +
>      return caps;
>  }
>  
> @@ -149,6 +159,8 @@ const VMStateDescription vmstate_spapr_caps = {
>  
>  static void spapr_allow_caps(sPAPRMachineState *spapr, Error **errp)
>  {
> +    PowerPCCPU *cpu = POWERPC_CPU(first_cpu);
> +    CPUPPCState *env = &cpu->env;
>      Error *err = NULL;
>  
>      if (spapr_has_cap(spapr, SPAPR_CAP_HTM)) {
> @@ -164,6 +176,14 @@ static void spapr_allow_caps(sPAPRMachineState *spapr, 
> Error **errp)
>          }
>      }
>  
> +    g_assert(env->insns_flags & PPC_ALTIVEC);
> +    if (spapr_has_cap(spapr, SPAPR_CAP_VSX)) {
> +        if (!(env->insns_flags2 & PPC2_VSX)) {
> +            error_setg(&err, "VSX support not available, try cap-vsx=off");
> +            goto out;
> +        }
> +    }
> +
>  out:
>      if (err) {
>          error_propagate(errp, err);
> @@ -175,6 +195,10 @@ static void spapr_enforce_caps(sPAPRMachineState *spapr, 
> Error **errp)
>      if (!spapr_has_cap(spapr, SPAPR_CAP_HTM)) {
>          /* TODO: Tell KVM not to allow HTM instructions */
>      }
> +
> +    if (!spapr_has_cap(spapr, SPAPR_CAP_VSX)) {
> +        /* TODO: Any way to disable VSX? */
> +    }
>  }
>  
>  void spapr_caps_reset(sPAPRMachineState *spapr)
> diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> index 9b80fe72ed..4c173cb40c 100644
> --- a/include/hw/ppc/spapr.h
> +++ b/include/hw/ppc/spapr.h
> @@ -59,6 +59,9 @@ typedef enum {
>  /* Hardware Transactional Memory */
>  #define SPAPR_CAP_HTM               0x0000000000000001ULL
>  
> +/* Vector Scalar Extensions */
> +#define SPAPR_CAP_VSX               0x0000000000000002ULL
> +
>  typedef struct sPAPRCapabilities sPAPRCapabilities;
>  struct sPAPRCapabilities {
>      uint64_t mask;