Re: [Qemu-devel] [PATCH 5/6] spapr: Handle VMX/VSX presence as an spapr capability flag

[Greg Kurz <groug@kaod.org>]

On Mon, 18 Dec 2017 20:20:23 +1100
David Gibson <david@gibson.dropbear.id.au> 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 <david@gibson.dropbear.id.au>
> ---

Reviewed-by: Greg Kurz <groug@kaod.org>

>  hw/ppc/spapr.c         | 20 +++++++++++---------
>  hw/ppc/spapr_caps.c    | 25 +++++++++++++++++++++++++
>  include/hw/ppc/spapr.h |  3 +++
>  3 files changed, 39 insertions(+), 9 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 86fc83f9c2..693dd6f7b3 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -557,14 +557,16 @@ 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)));
> +    } else {
> +        _FDT((fdt_setprop_cell(fdt, offset, "ibm,vmx", 1)));
>      }
>  
>      /* Advertise DFP (Decimal Floating Point) if available
> @@ -3832,7 +3834,7 @@ static void spapr_machine_class_init(ObjectClass *oc, void *data)
>       */
>      mc->numa_mem_align_shift = 28;
>  
> -    smc->default_caps = spapr_caps(0);
> +    smc->default_caps = spapr_caps(SPAPR_CAP_VSX);
>      spapr_caps_add_properties(smc, &error_abort);
>  }
>  
> @@ -3914,7 +3916,7 @@ static void spapr_machine_2_11_class_options(MachineClass *mc)
>      sPAPRMachineClass *smc = SPAPR_MACHINE_CLASS(mc);
>  
>      spapr_machine_2_12_class_options(mc);
> -    smc->default_caps = spapr_caps(SPAPR_CAP_HTM);
> +    smc->default_caps = spapr_caps(SPAPR_CAP_HTM | SPAPR_CAP_VSX);
>      SET_MACHINE_COMPAT(mc, SPAPR_COMPAT_2_11);
>  }
>  
> diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
> index a8c726a88b..da066aec8f 100644
> --- a/hw/ppc/spapr_caps.c
> +++ b/hw/ppc/spapr_caps.c
> @@ -57,6 +57,19 @@ static void cap_htm_allow(sPAPRMachineState *spapr, Error **errp)
>      }
>  }
>  
> +static void cap_vsx_allow(sPAPRMachineState *spapr, Error **errp)
> +{
> +    PowerPCCPU *cpu = POWERPC_CPU(first_cpu);
> +    CPUPPCState *env = &cpu->env;
> +
> +    /* Allowable CPUs in spapr_cpu_core.c should already have gotten
> +     * rid of anything that doesn't do VMX */
> +    g_assert(env->insns_flags & PPC_ALTIVEC);
> +    if (!(env->insns_flags2 & PPC2_VSX)) {
> +        error_setg(errp, "VSX support not available, try cap-vsx=off");
> +    }
> +}
> +
>  static sPAPRCapabilityInfo capability_table[] = {
>      {
>          .name = "htm",
> @@ -65,6 +78,13 @@ static sPAPRCapabilityInfo capability_table[] = {
>          .allow = cap_htm_allow,
>          /* TODO: add cap_htm_disallow */
>      },
> +    {
> +        .name = "vsx",
> +        .description = "Allow Vector Scalar Extensions (VSX)",
> +        .flag = SPAPR_CAP_VSX,
> +        .allow = cap_vsx_allow,
> +        /* TODO: add cap_vsx_disallow */
> +    },
>  };
>  
>  static sPAPRCapabilities default_caps_with_cpu(sPAPRMachineState *spapr,
> @@ -81,6 +101,11 @@ static sPAPRCapabilities default_caps_with_cpu(sPAPRMachineState *spapr,
>          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;
>  }
>  
> diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> index 5c85f39c3b..148a03d189 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;