Re: [RFC PATCH v7 10/18] target/arm/kvm: Retrieve writable ID reg map

> On 26 Jul 2026, at 8:59 PM, Eric Auger <[email protected]> wrote:
> 
> !-------------------------------------------------------------------|
>  CAUTION: External Email
> 
> |-------------------------------------------------------------------!
> 
> From: Cornelia Huck <[email protected]>
> 
> Add an helper to retrieve the writable id reg bitmask.
> 
> then retrieve the writable_map array and dispatch it in the writable_map
> fields of arm64_id_regs[] elements.
> 
> Signed-off-by: Eric Auger <[email protected]>
> Signed-off-by: Cornelia Huck <[email protected]>
> 
> ---
> 
> v5 -> v6
> - use arm64_id_regs[].writable_map
> - squashed 2 commits (helper + user)
> - removed stub
> ---
> target/arm/kvm.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 61 insertions(+)
> 
> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
> index d40a6a9859..d270f9870e 100644
> --- a/target/arm/kvm.c
> +++ b/target/arm/kvm.c
> @@ -28,6 +28,7 @@
> #include "kvm_arm.h"
> #include "cpu.h"
> #include "cpu-sysregs.h"
> +#include "cpu-idregs.h"
> #include "trace.h"
> #include "internals.h"
> #include "hw/pci/pci.h"
> @@ -51,6 +52,7 @@ const KVMCapabilityInfo kvm_arch_required_capabilities[] = {
> static bool cap_has_mp_state;
> static bool cap_has_inject_serror_esr;
> static bool cap_has_inject_ext_dabt;
> +static bool writable_map_dispatched;
> 
> /**
>  * ARMHostCPUFeatures: information about the host CPU (identified
> @@ -273,6 +275,21 @@ static uint32_t kvm_arm_sve_get_vls(int fd)
>     return vls[0] & MAKE_64BIT_MASK(0, ARM_MAX_VQ);
> }
> 
> +static int kvm_feature_idx_to_idregs_idx(int kidx)
> +{
> +    int op1, crm, op2;
> +    ARMSysRegs sysreg;
> +
> +    op1 = kidx / 64;
> +    if (op1 == 2) {
> +        op1 = 3;
> +    }
> +    crm = (kidx % 64) / 8;
> +    op2 = kidx % 8;
> +    sysreg = ENCODE_ID_REG(3, op1, 0, crm, op2);
> +    return get_sysreg_idx(sysreg);
> +}
> +
> static void kvm_arm_get_host_cpu_features(ARMHostCPUFeatures *ahcf)
> {
>     /* Identify the feature bits corresponding to the host CPU, and
> @@ -480,9 +497,53 @@ static void kvm_arm_get_host_cpu_features(ARMHostCPUFeatures *ahcf)
>     ahcf->features = features;
> }
> 
> +static int kvm_arm_get_writable_id_regs(uint64_t *idregmap)
> +{
This should probably be gated with kvm_enabled()?

qtest-aarch64/device-introspect-test fails on my aarch64 devvm, which does not
have KVM.

> +    int cap_writable_id_regs;
> +    struct reg_mask_range range = {
> +        .range = KVM_ARM_FEATURE_ID_RANGE,
> +        .reserved = {0},
> +        .addr = (uint64_t)idregmap,
> +    };
> +
> +    cap_writable_id_regs =
> +        kvm_check_extension(kvm_state, KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES);
> +
> +    if (!cap_writable_id_regs ||
> +        !(cap_writable_id_regs & (1 << KVM_ARM_FEATURE_ID_RANGE))) {
> +        return -ENOSYS;
> +    }
> +
> +    if (kvm_vm_ioctl(kvm_state, KVM_ARM_GET_REG_WRITABLE_MASKS, &range)) {
> +        return -errno;
> +    }
> +    return 0;
> +}
> +
> void kvm_arm_set_cpu_features_from_host(ARMCPU *cpu)
> {
>     CPUARMState *env = &cpu->env;
> +    uint64_t *writable_map;
> +
> +    writable_map = g_new(uint64_t, KVM_ARM_FEATURE_ID_RANGE_SIZE);
> +
> +    if (!writable_map_dispatched &&
> +        !kvm_arm_get_writable_id_regs(writable_map)) {
> +        for (int i = 0; i < KVM_ARM_FEATURE_ID_RANGE_SIZE; i++) {
> +            uint64_t mask = writable_map[i];
> +
> +            if (mask) {
> +                int idx = kvm_feature_idx_to_idregs_idx(i);
> +
> +                if (idx < 0 || idx > ARRAY_SIZE(arm64_id_regs)) {
> +                    continue;
> +                }
> +                arm64_id_regs[idx].writable_mask = mask;
> +            }
> +        }
> +        writable_map_dispatched = true;
> +    }
> +    g_free(writable_map);
> 
>     if (!arm_host_cpu_features.dtb_compatible) {
>         kvm_arm_get_host_cpu_features(&arm_host_cpu_features);
> -- 
> 2.53.0
>