Re: [PATCH v2 12/12] target/arm: Allow users to set the number of VFP registers

[Mads Ynddal]

Sorry, if this has already been acknowledged, but I couldn't find it on the
mailinglist.

This commit seems to break compatibility with macOS accelerator hvf when
virtualizing ARM CPUs.

It breaks the VM on boot-up with the message "ARM CPUs must have both VFP-D32
and Neon or neither". I haven't looked into what VFP-D32 and Neon are, but the
same VM worked on earlier versions of QEMU.

It can be reproduced with the following:

qemu-system-aarch64 \
  -nodefaults \
  -display "none" \
  -machine "virt" \
  -accel "hvf" \
  -cpu "host" \
  -serial "mon:stdio"
qemu-system-aarch64: ARM CPUs must have both VFP-D32 and Neon or neither


If you fix/work on this issue in a separate thread/patch, you can add
reported-by, so I'll automatically follow and help test it:

Reported-by: Mads Ynddal <mads@ynddal.dk>

—
Mads Ynddal

> On 7 Jun 2023, at 06.39, Cédric Le Goater <clg@kaod.org> wrote:
> 
> Cortex A7 CPUs with an FPU implementing VFPv4 without NEON support
> have 16 64-bit FPU registers and not 32 registers. Let users set the
> number of VFP registers with a CPU property.
> 
> The primary use case of this property is for the Cortex A7 of the
> Aspeed AST2600 SoC.
> 
> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> ---
> target/arm/cpu.h        |  2 ++
> hw/arm/aspeed_ast2600.c |  2 ++
> target/arm/cpu.c        | 32 ++++++++++++++++++++++++++++++++
> 3 files changed, 36 insertions(+)
> 
> diff --git a/target/arm/cpu.h b/target/arm/cpu.h
> index d469a2637b32..79f1a96ddf39 100644
> --- a/target/arm/cpu.h
> +++ b/target/arm/cpu.h
> @@ -916,6 +916,8 @@ struct ArchCPU {
>     bool has_pmu;
>     /* CPU has VFP */
>     bool has_vfp;
> +    /* CPU has 32 VFP registers */
> +    bool has_vfp_d32;
>     /* CPU has Neon */
>     bool has_neon;
>     /* CPU has M-profile DSP extension */
> diff --git a/hw/arm/aspeed_ast2600.c b/hw/arm/aspeed_ast2600.c
> index 1bf12461481c..a8b3a8065a11 100644
> --- a/hw/arm/aspeed_ast2600.c
> +++ b/hw/arm/aspeed_ast2600.c
> @@ -316,6 +316,8 @@ static void aspeed_soc_ast2600_realize(DeviceState *dev, 
> Error **errp)
>                                 &error_abort);
>         object_property_set_bool(OBJECT(&s->cpu[i]), "neon", false,
>                                 &error_abort);
> +        object_property_set_bool(OBJECT(&s->cpu[i]), "vfp-d32", false,
> +                                &error_abort);
>         object_property_set_link(OBJECT(&s->cpu[i]), "memory",
>                                  OBJECT(s->memory), &error_abort);
> 
> diff --git a/target/arm/cpu.c b/target/arm/cpu.c
> index 5182ed0c9113..74fe6ae78192 100644
> --- a/target/arm/cpu.c
> +++ b/target/arm/cpu.c
> @@ -1275,6 +1275,9 @@ static Property arm_cpu_cfgend_property =
> static Property arm_cpu_has_vfp_property =
>             DEFINE_PROP_BOOL("vfp", ARMCPU, has_vfp, true);
> 
> +static Property arm_cpu_has_vfp_d32_property =
> +            DEFINE_PROP_BOOL("vfp-d32", ARMCPU, has_vfp_d32, true);
> +
> static Property arm_cpu_has_neon_property =
>             DEFINE_PROP_BOOL("neon", ARMCPU, has_neon, true);
> 
> @@ -1406,6 +1409,22 @@ void arm_cpu_post_init(Object *obj)
>         }
>     }
> 
> +    if (cpu->has_vfp && cpu_isar_feature(aa32_simd_r32, cpu)) {
> +        cpu->has_vfp_d32 = true;
> +        if (!kvm_enabled()) {
> +            /*
> +             * The permitted values of the SIMDReg bits [3:0] on
> +             * Armv8-A are either 0b0000 and 0b0010. On such CPUs,
> +             * make sure that has_vfp_d32 can not be set to false.
> +             */
> +            if (!(arm_feature(&cpu->env, ARM_FEATURE_V8) &&
> +                  !arm_feature(&cpu->env, ARM_FEATURE_M))) {
> +                qdev_property_add_static(DEVICE(obj),
> +                                         &arm_cpu_has_vfp_d32_property);
> +            }
> +        }
> +    }
> +
>     if (arm_feature(&cpu->env, ARM_FEATURE_NEON)) {
>         cpu->has_neon = true;
>         if (!kvm_enabled()) {
> @@ -1672,6 +1691,19 @@ static void arm_cpu_realizefn(DeviceState *dev, Error 
> **errp)
>         return;
>     }
> 
> +    if (cpu->has_vfp_d32 != cpu->has_neon) {
> +        error_setg(errp, "ARM CPUs must have both VFP-D32 and Neon or 
> neither");
> +        return;
> +    }
> +
> +   if (!cpu->has_vfp_d32) {
> +        uint32_t u;
> +
> +        u = cpu->isar.mvfr0;
> +        u = FIELD_DP32(u, MVFR0, SIMDREG, 1); /* 16 registers */
> +        cpu->isar.mvfr0 = u;
> +    }
> +
>     if (!cpu->has_vfp) {
>         uint64_t t;
>         uint32_t u;
> -- 
> 2.40.1
> 
> 
>