Re: [RFC PATCH v3 06/19] target/arm/kvm: Read all ID registers from KVM

> On 22 Jul 2026, at 12:24 PM, Eric Auger <[email protected]> wrote:
> 
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> 
> On 7/16/26 11:38 PM, Khushit Shah wrote:
>> Try to read all the ID registers from KVM. KVM may return ENOENT, for
>> ID registers it does not support reading from userspace. Ignore those
>> registers. As this list is small, in future we may anyway want to
>> handle those register specially.
>> 
>> We want to do this so that we can expose more or less complete
>> host feature view to management stack, as for migration, we want both
>> writable and non-writable ID registers to match.
>> 
>> Also remove the unused 'cpu' args throughout the call-chain.
> I will integrate this fix in the parent series

Acked.

>> 
>> Signed-off-by: Khushit Shah <[email protected]>
>> ---
>> target/arm/kvm.c | 38 +++++++++++++++++++-------------------
>> 1 file changed, 19 insertions(+), 19 deletions(-)
>> 
>> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
>> index 63b3727e05..42ff731f6e 100644
>> --- a/target/arm/kvm.c
>> +++ b/target/arm/kvm.c
>> @@ -292,14 +292,14 @@ static int kvm_feature_idx_to_idregs_idx(int kidx)
>> }
>> 
>> /*
>> - * get_host_cpu_idregs: Read all the writable ID reg host values
>> + * get_host_cpu_idregs: Read all the ID reg host values
>>  *
>> - * Need to be called once the writable mask has been populated
>> - * Note we may want to read all the known id regs but some of them are not
>> - * writable and return an error, hence the choice of reading only those which
>> - * are writable. Those are also readable!
>> + * We currently try to read all the registers. If the read fails with
>> + * ENOENT (KVM does not identify the ID register) ignore it.
>> + * Some of the registers that errored with ENOENT will anyway need special
>> + * handling.
>>  */
>> -static int get_host_cpu_idregs(ARMCPU *cpu, int fd, ARMHostCPUFeatures *ahcf)
>> +static int get_host_cpu_idregs(int fd, ARMHostCPUFeatures *ahcf)
>> {
>>     int err = 0;
>>     int i;
>> @@ -310,26 +310,26 @@ static int get_host_cpu_idregs(ARMCPU *cpu, int fd, ARMHostCPUFeatures *ahcf)
>>         uint64_t *reg;
>>         int ret;
>> 
>> -        if (!sysregdesc->writable_mask) {
>> -            continue;
>> -        }
>> -
>>         reg = &ahcf->isar.idregs[i];
>>         ret = read_sys_reg64(fd, reg, idregs_sysreg_to_kvm_reg(sysreg));
>> -        trace_get_host_cpu_idregs(sysregdesc->name, *reg);
>>         if (ret) {
>> -            error_report("%s error reading value of host %s register (%m)",
>> -                         __func__, sysregdesc->name);
>> -
>> -            err = ret;
>> +            if (errno == ENOENT) {
>> +                warn_report("%s error reading value of host %s register "
>> +                                "(ENOENT)", __func__, sysregdesc->name);
> if there are expected cases where the read is failing I don't think we
> want to warn. Add a trace point instead.

Acked, will change to tracepoint in v4.

>> +            } else {
>> +                error_report("%s error reading value of host %s register"
>> +                            " (%m)", __func__, sysregdesc->name);
>> +                err = ret;
>> +            }
>>         }
>> +        trace_get_host_cpu_idregs(sysregdesc->name, *reg);
>>     }
>>     return err;
>> }
>> 
>> 
>> static void
>> -kvm_arm_get_host_cpu_features(ARMCPU *cpu, ARMHostCPUFeatures *ahcf)
>> +kvm_arm_get_host_cpu_features(ARMHostCPUFeatures *ahcf)
>> {
>>     /* Identify the feature bits corresponding to the host CPU, and
>>      * fill out the ARMHostCPUClass fields accordingly. To do this
>> @@ -415,8 +415,8 @@ kvm_arm_get_host_cpu_features(ARMCPU *cpu, ARMHostCPUFeatures *ahcf)
>>         SET_IDREG(&ahcf->isar, ID_AA64PFR0, 0x00000011); /* EL1&0, AArch64 only */
>>         err = 0;
>>     } else {
>> -        /* Make sure all writable ID reg values are initialized */
>> -        err |= get_host_cpu_idregs(cpu, fd, ahcf);
>> +        /* Try to read all the ID registers */
>> +        err |= get_host_cpu_idregs(fd, ahcf);
>> 
>>         /*
>>          * temporarily override the CLIDR_EL1 value since some host values
>> @@ -595,7 +595,7 @@ void kvm_arm_set_cpu_features_from_host(ARMCPU *cpu)
>>     g_free(writable_map);
>> 
>>     if (!arm_host_cpu_features.dtb_compatible) {
>> -        kvm_arm_get_host_cpu_features(cpu, &arm_host_cpu_features);
>> +        kvm_arm_get_host_cpu_features(&arm_host_cpu_features);
>>     }
>> 
>>     cpu->kvm_target = arm_host_cpu_features.target;
> Thanks
> 
> Eric