On 9/21/26 2:32 PM, Khushit Shah wrote:
> When QEMU queries the host ISAR via KVM, it uses a scratch vCPU
> initialized with all host-supported features (like SVE and PAuth).
So I expect this patch is not needed anymore after rebase on v9/v10
Eric
>
> KVM exposes the corresponding ID register fields (e.g.,
> ID_AA64ISAR1_EL1.APA) as read-only. The supported-values list only
> shows the enabled host value (e.g., 5). This falsely flags a model as
> blocked, even though the features can be masked by omitting those flags
> during vCPU initialization. For these fields, even if KVM reports them as
> non-writable, we explicitly append "0" to the list of supported values
> if the host value is non-zero.
>
> Signed-off-by: Khushit Shah <[email protected]>
> ---
> target/arm/kvm.c | 25 +++++++++++++++++++++++++
> 1 file changed, 25 insertions(+)
>
> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
> index 77b1bcdf55..0b5fb49dff 100644
> --- a/target/arm/kvm.c
> +++ b/target/arm/kvm.c
> @@ -1346,6 +1346,24 @@ static void ranges_add(GArray *ranges, uint64_t min,
> uint64_t max)
> g_array_append_val(ranges, r);
> }
>
> +/*
> + * Some fields are reported as non-writable by KVM but can still be
> + * disabled by not requesting the corresponding vCPU init feature flag
> + * (SVE, pauth). The supported values are derived from a scratch vCPU
> + * whose init flags may differ from the real vCPU, so for these fields
> + * "off" (0) is a supported value in addition to the host value.
> + */
> +static bool kvm_arm_field_off_via_vcpu_flags(const ARM64SysRegField *field)
> +{
> + return field_matches(field, ID_AA64PFR0_EL1_IDX, "SVE") ||
> + field_matches(field, ID_AA64ISAR1_EL1_IDX, "APA") ||
> + field_matches(field, ID_AA64ISAR1_EL1_IDX, "API") ||
> + field_matches(field, ID_AA64ISAR1_EL1_IDX, "GPA") ||
> + field_matches(field, ID_AA64ISAR1_EL1_IDX, "GPI") ||
> + field_matches(field, ID_AA64ISAR2_EL1_IDX, "APA3") ||
> + field_matches(field, ID_AA64ISAR2_EL1_IDX, "GPA3");
> +}
> +
> void kvm_arm_field_get_supported_values(const ARM64SysRegField *field,
> const ARMISARegisters *host_isar,
> ArmFieldValueSet **value_set)
> @@ -1358,6 +1376,13 @@ void kvm_arm_field_get_supported_values(const
> ARM64SysRegField *field,
> /* A non-writable field can only ever hold the host value. */
> if (!arm_field_is_writable(field)) {
> ranges_add(ranges, host, host);
> + /*
> + * ...unless it is one of the fields that can still be turned off
> + * via vCPU init flags, in which case "off" (0) is also supported.
> + */
> + if (host != 0 && kvm_arm_field_off_via_vcpu_flags(field)) {
> + ranges_add(ranges, 0, 0);
> + }
> goto done;
> }
>
