Re: [PATCH v2 2/3] spapr: Add SPAPR_CAP_AIL_MODE_3 for AIL mode 3 support for H_SET_MODE hcall

[David Gibson]

On Wed, Feb 16, 2022 at 04:39:02PM +1000, Nicholas Piggin wrote:
> The behaviour of the Address Translation Mode on Interrupt resource is
> not consistently supported by all CPU versions or all KVM versions:
> KVM-HV does not support mode 2, and does not support mode 3 on POWER7 or
> early POWER9 processesors. KVM PR only supports mode 0. TCG supports all
> modes (0, 2, 3). This leads to inconsistencies in guest behaviour and
> could cause problems migrating guests.
> 
> This was not noticable for Linux guests for a long time because the
> kernel only uses modes 0 and 3, and it used to consider AIL-3 to be
> advisory in that it would always keep the AIL-0 vectors around. Recent
> Linux guests depend on the AIL mode working as specified in order to
> support the SCV facility interrupt. If AIL-3 can not be provided, then
> Linux must be given an error so it can disable the SCV facility, rather
> than silently failing.
> 
> Add the ail-mode-3 capability to specify that AIL-3 is supported. AIL-0
> is implied as the baseline, and AIL-2 is no longer supported by spapr.
> AIL-2 is not known to be used by any software, but support in TCG could
> be restored with an ail-mode-2 capability quite easily if a regression
> is reported.
> 
> Modify the H_SET_MODE Address Translation Mode on Interrupt resource
> handler to check capabilities and correctly return error if not
> supported.
> 
> A heuristic is added for KVM to determine AIL-3 support before the
> introduction of a new KVM CAP, because blanket disabling AIL-3 has too
> much performance cost.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

[snip]
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index dc93b99189..1338c41f8f 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -2563,6 +2563,35 @@ int kvmppc_has_cap_rpt_invalidate(void)
>      return cap_rpt_invalidate;
>  }
>  
> +bool kvmppc_supports_ail_3(void)
> +{
> +    PowerPCCPUClass *pcc = kvm_ppc_get_host_cpu_class();
> +
> +    /*
> +     * KVM PR only supports AIL-0
> +     */
> +    if (kvmppc_is_pr(kvm_state)) {
> +        return 0;
> +    }
> +
> +    /*
> +     * KVM HV hosts support AIL-3 on POWER8 and above, except for radix
> +     * mode on some early POWER9s.
> +     */
> +    if (!(pcc->insns_flags2 & PPC2_ISA207S)) {
> +        return 0;
> +    }
> +
> +    /* These tests match the CPU_FTR_P9_RADIX_PREFETCH_BUG flag in Linux */
> +    if (((pcc->pvr & 0xffffff00) == CPU_POWERPC_POWER9_DD1) ||
> +        ((pcc->pvr & 0xffffff00) == CPU_POWERPC_POWER9_DD20) ||
> +        ((pcc->pvr & 0xffffff00) == CPU_POWERPC_POWER9_DD21)) {
> +        return 0;
> +    }

Deducing what KVM supports rather than getting it to tell us
explicitly with a cap is usually frowned upon.  However, given the
earlier discussion, I'm satisfied that this is the least bad available
option, at least for now.


> +
> +    return 1;
> +}
> +
>  PowerPCCPUClass *kvm_ppc_get_host_cpu_class(void)
>  {
>      uint32_t host_pvr = mfpvr();
> diff --git a/target/ppc/kvm_ppc.h b/target/ppc/kvm_ppc.h
> index ee9325bf9a..7bba26d1da 100644
> --- a/target/ppc/kvm_ppc.h
> +++ b/target/ppc/kvm_ppc.h
> @@ -73,6 +73,7 @@ int kvmppc_set_cap_nested_kvm_hv(int enable);
>  int kvmppc_get_cap_large_decr(void);
>  int kvmppc_enable_cap_large_decr(PowerPCCPU *cpu, int enable);
>  int kvmppc_has_cap_rpt_invalidate(void);
> +bool kvmppc_supports_ail_3(void);
>  int kvmppc_enable_hwrng(void);
>  int kvmppc_put_books_sregs(PowerPCCPU *cpu);
>  PowerPCCPUClass *kvm_ppc_get_host_cpu_class(void);
> @@ -393,6 +394,11 @@ static inline int kvmppc_has_cap_rpt_invalidate(void)
>      return false;
>  }
>  
> +static inline bool kvmppc_supports_ail_3(void)
> +{
> +    return false;
> +}
> +
>  static inline int kvmppc_enable_hwrng(void)
>  {
>      return -1;

-- 
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

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