Re: [PATCH 1/2] target/arm: Add cpu property to control pauth

[Andrew Jones]

On Tue, Aug 11, 2020 at 11:53:38PM -0700, Richard Henderson wrote:
> The crypto overhead of emulating pauth can be significant for
> some workloads.  Add an enumeration property that allows the
> feature to be turned off, on with the architected algorithm,
> or on with an implementation defined algorithm.
> 
> The architected algorithm is quite expensive to emulate;
> using another algorithm may allow hardware acceleration.
> 
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  target/arm/cpu64.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 64 insertions(+)
> 
> diff --git a/target/arm/cpu64.c b/target/arm/cpu64.c
> index dd696183df..3181d0e2f8 100644
> --- a/target/arm/cpu64.c
> +++ b/target/arm/cpu64.c
> @@ -572,6 +572,69 @@ void aarch64_add_sve_properties(Object *obj)
>      }
>  }
>  
> +static const char * const pauth_names[] = {
> +    "off", "impdef", "arch"
> +};

Hi Richard,

Please add three boolean properties, rather than one enum:

pauth:            enable support of the pauth feature
pauth-fast:       enable QEMU's fast non-cryptographic hash for pauth
                  (pauth must be enabled)
pauth-arch:       enable the architected algorithm for pauth
                  (pauth must be enabled)

These booleans can then be added to the cpu feature probing list used by
qmp_query_cpu_model_expansion()

> +
> +static const QEnumLookup pauth_lookup = {
> +    .array = pauth_names,
> +    .size = ARRAY_SIZE(pauth_names)
> +};
> +
> +static int cpu_arm_get_pauth(Object *obj, Error **errp)
> +{
> +    ARMCPU *cpu = ARM_CPU(obj);
> +    int value;
> +
> +    /* We will always set GPA+APA and GPI+API to the same value. */
> +    if (FIELD_EX64(cpu->isar.id_aa64isar1, ID_AA64ISAR1, APA)) {
> +        value = 2;
> +    } else if (FIELD_EX64(cpu->isar.id_aa64isar1, ID_AA64ISAR1, API)) {
> +        value = 1;
> +    } else {
> +        value = 0;
> +    }
> +    return value;
> +}
> +
> +static void cpu_arm_set_pauth(Object *obj, int value, Error **errp)
> +{
> +    ARMCPU *cpu = ARM_CPU(obj);
> +    uint64_t t;
> +
> +    /* TODO: Handle HaveEnhancedPAC, HaveEnhancedPAC2, HaveFPAC. */
> +    t = cpu->isar.id_aa64isar1;
> +    switch (value) {
> +    case 0:
> +        t = FIELD_DP64(t, ID_AA64ISAR1, APA, 0);
> +        t = FIELD_DP64(t, ID_AA64ISAR1, API, 0);
> +        t = FIELD_DP64(t, ID_AA64ISAR1, GPA, 0);
> +        t = FIELD_DP64(t, ID_AA64ISAR1, GPI, 0);
> +        break;
> +    case 1:
> +        t = FIELD_DP64(t, ID_AA64ISAR1, APA, 0);
> +        t = FIELD_DP64(t, ID_AA64ISAR1, API, 1);
> +        t = FIELD_DP64(t, ID_AA64ISAR1, GPA, 0);
> +        t = FIELD_DP64(t, ID_AA64ISAR1, GPI, 1);
> +        break;
> +    case 2:
> +        t = FIELD_DP64(t, ID_AA64ISAR1, APA, 1);
> +        t = FIELD_DP64(t, ID_AA64ISAR1, API, 0);
> +        t = FIELD_DP64(t, ID_AA64ISAR1, GPA, 1);
> +        t = FIELD_DP64(t, ID_AA64ISAR1, GPI, 0);
> +        break;
> +    default:
> +        g_assert_not_reached();
> +    }
> +    cpu->isar.id_aa64isar1 = t;
> +}
> +
> +static void aarch64_add_pauth_properties(Object *obj)
> +{
> +    object_property_add_enum(obj, "pauth", "ARMPauthKind", &pauth_lookup,
> +                             cpu_arm_get_pauth, cpu_arm_set_pauth);
> +}
> +
>  /* -cpu max: if KVM is enabled, like -cpu host (best possible with this 
> host);
>   * otherwise, a CPU with as many features enabled as our emulation supports.
>   * The version of '-cpu max' for qemu-system-arm is defined in cpu.c;
> @@ -720,6 +783,7 @@ static void aarch64_max_initfn(Object *obj)
>  #endif
>      }
>  
> +    aarch64_add_pauth_properties(obj);

We don't yet support enabling pauth on KVM accelerated guests, so this
call should be in the !kvm_enabled() part of this function.

>      aarch64_add_sve_properties(obj);
>      object_property_add(obj, "sve-max-vq", "uint32", cpu_max_get_sve_max_vq,
>                          cpu_max_set_sve_max_vq, NULL, NULL);
> -- 
> 2.25.1
> 
>

Thanks,
drew