> -----Original Message-----
> From: Eric Auger <[email protected]>
> Sent: 03 May 2026 08:34
> To: [email protected]; [email protected]; qemu-
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected];
> [email protected]; [email protected]; Shameer Kolothum Thodi
> <[email protected]>; [email protected]
> Cc: [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]
> Subject: [PATCH v4 15/17] arm/cpu: Expose writable ID reg field properties on
> the kvm host vcpu model
>
> External email: Use caution opening links or attachments
>
>
> If the host supports KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES and
> KVM_ARM_GET_REG_WRITABLE_MASKS ioctl successfully retrieved the mask
> of writable fields for all ID regs, expose uint64 SYSREG properties
> for all the writable ID reg fields exposed by the host kernel which
> can be matched in target/arm/cpu-sysreg-properties.c.
>
> Properties are named SYSREG_<REG>_<FIELD> with REG and FIELD
> being those used in linux arch/arm64/tools/sysreg or in the AARCHMRS
> Registers.json.
>
> This is achieved by matching the writable fields retrieved from the
> host kernel against the generated description of ID regs and their
> fields in target/arm/cpu-sysreg-properties.c.
>
> An example of invocation is:
> -cpu host,SYSREG_ID_AA64ISAR0_EL1_DP=0x0
> which sets DP field of ID_AA64ISAR0_EL1 to 0.
>
> [CH: add properties to the host model instead of introducing a new
> "custom" model]
> Signed-off-by: Eric Auger <[email protected]>
> Signed-off-by: Cornelia Huck <[email protected]>
> ---
> target/arm/cpu.c | 12 ++++++++++++
> target/arm/cpu64.c | 23 ++++++++++++++++++++++-
> 2 files changed, 34 insertions(+), 1 deletion(-)
>
> diff --git a/target/arm/cpu.c b/target/arm/cpu.c
> index 10feb639c4..10ce4eb0cb 100644
> --- a/target/arm/cpu.c
> +++ b/target/arm/cpu.c
> @@ -1824,6 +1824,18 @@ static void arm_cpu_realizefn(DeviceState *dev,
> Error **errp)
> return;
> }
>
> + /*
> + * If we failed to retrieve the set of writable ID registers for the
> "host"
> + * CPU model, report it here. No error if the interface for discovering
> + * writable ID registers is not available.
> + * In case we did get the set of writable ID registers, set the features
> to
> + * the configured values here and perform some sanity checks.
> + */
> + if (cpu->writable_id_regs_status == WRITABLE_ID_REGS_FAILED) {
> + error_setg(errp, "Failed to discover writable id registers");
> + return;
> + }
> +
> if (!cpu->gt_cntfrq_hz) {
> /*
> * 0 means "the board didn't set a value, use the default". (We also
> diff --git a/target/arm/cpu64.c b/target/arm/cpu64.c
> index 1b3d3fb245..d66cb00a21 100644
> --- a/target/arm/cpu64.c
> +++ b/target/arm/cpu64.c
> @@ -852,6 +852,8 @@ static void
> kvm_arm_set_cpreg_mig_tolerances(ARMCPU *cpu)
> static void aarch64_host_initfn(Object *obj)
> {
> ARMCPU *cpu = ARM_CPU(obj);
> + bool expose_id_regs = true;
> + int ret;
The above will be unused for non KVM builds.
>
> #if defined(CONFIG_NITRO)
> if (nitro_enabled()) {
> @@ -862,8 +864,27 @@ static void aarch64_host_initfn(Object *obj)
>
> #if defined(CONFIG_KVM)
> kvm_arm_set_cpreg_mig_tolerances(cpu);
> - kvm_arm_set_cpu_features_from_host(cpu, false);
> +
> + cpu->writable_map = g_malloc(sizeof(IdRegMap));
Does kernel clear the buffer before setting writable mask?
> + /* discover via KVM_ARM_GET_REG_WRITABLE_MASKS */
> + ret = kvm_arm_get_writable_id_regs(cpu, cpu->writable_map);
> + if (ret == -ENOSYS) {
> + /* legacy: continue without writable id regs */
> + expose_id_regs = false;
> + } else if (ret) {
> + /* function will have marked an error */
> + return;
> + }
Should free and NULL writable_map for both error paths above.
Thanks,
Shameer
> +
> + kvm_arm_set_cpu_features_from_host(cpu, expose_id_regs);
> aarch64_add_sve_properties(obj);
> +
> + if (expose_id_regs) {
> + /* generate SYSREG properties according to writable masks */
> + kvm_arm_expose_idreg_properties(cpu, arm64_id_regs);
> + }
> +
> #elif defined(CONFIG_HVF)
> hvf_arm_set_cpu_features_from_host(cpu);
> #elif defined(CONFIG_WHPX)
> --
> 2.53.0
