Re: [RFC PATCH v3 07/19] target/arm/kvm: handle special ID registers cases when reading from KVM

> On 22 Jul 2026, at 6:02 PM, Eric Auger <[email protected]> wrote:
> 
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> Hi Khushit,
> 
> On 7/16/26 11:38 PM, Khushit Shah wrote:
>> The generic KVM_GET_ONE_REG loop cannot correctly read a handful of
>> ID registers, so specially handle those:
>> 
>> - DCZID_EL0: No fine grain trap exists, hence not tracked by KVM.
>>  It is EL0-readable and untrapped, so read it directly with MRS.
>> 
>> - CCSIDR_EL1/CCSIDR2_EL1: not plain ID registers, their value is
>>  selected by CSSELR_EL1. KVM only exposes them via the DEMUX API
>>  (KVM_REG_ARM_DEMUX_ID_CCSIDR), so there is no single value to read
>>  here. Skip them.
>> 
>> - SMIDR_EL1: KVM does not support SME, so there is no meaningful host
>>  value. Skip it.
>> 
>> - GMID_EL1: not in KVM's sys_reg_descs[] and, being an EL1 register,
>>  cannot be read from userspace via MRS either. The guest reads the
>>  raw host value; this is a genuine miss when MTE is enabled.
>> 
>> Signed-off-by: Khushit Shah <[email protected]>
>> ---
>> target/arm/kvm.c | 53 ++++++++++++++++++++++++++++++++++++++++--------
>> 1 file changed, 44 insertions(+), 9 deletions(-)
>> 
>> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
>> index 42ff731f6e..6974e5c551 100644
>> --- a/target/arm/kvm.c
>> +++ b/target/arm/kvm.c
>> @@ -311,15 +311,50 @@ static int get_host_cpu_idregs(int fd, ARMHostCPUFeatures *ahcf)
>>         int ret;
>> 
>>         reg = &ahcf->isar.idregs[i];
>> -        ret = read_sys_reg64(fd, reg, idregs_sysreg_to_kvm_reg(sysreg));
>> -        if (ret) {
>> -            if (errno == ENOENT) {
>> -                warn_report("%s error reading value of host %s register "
>> -                                "(ENOENT)", __func__, sysregdesc->name);
>> -            } else {
>> -                error_report("%s error reading value of host %s register"
>> -                            " (%m)", __func__, sysregdesc->name);
>> -                err = ret;
>> +        switch (i) {
>> +        case DCZID_EL0_IDX:
>> +            /*
>> +             * DCZID_EL0 is not in KVM's sys_reg_descs[], so
>> +             * KVM_GET_ONE_REG will fail. Read it directly from
>> +             * hardware since KVM doesn't trap guest reads of it.
>> +             */
>> +            asm volatile("mrs %0, DCZID_EL0" : "=r" (*reg));
>> +            break;
>> +        case CCSIDR_EL1_IDX:
>> +        case CCSIDR2_EL1_IDX:
>> +            /*
>> +             * CCSIDR_EL1 is not a plain ID register: the value it
>> +             * returns is selected by CSSELR_EL1, so there is no single
>> +             * value to capture here. It is exposed to userspace only via
>> +             * the DEMUX API (KVM_REG_ARM_DEMUX_ID_CCSIDR), indexed by
>> +             * CSSELR. Skip reading it here as there is no meaningful host
>> +             * value to read.
>> +             */
> I think you should rely on prerequisite
> [PATCH v4 3/3] arm/kvm: get demuxed ID registers from kvm
Currently I am just skipping those as anyway KVM exposes a very
barebone Cache topology.

Agreed on using DEMUX regs, in case we want to support exopsing some other
“config”.

Also on this point, I don’t know if CCSIDR_EL1/CCSIDR2_EL1 should
be part of idregs[], we already have a ccsidr[], maybe this can be fixed
in your series.

>> +            break;
>> +        case SMIDR_EL1_IDX:
>> +            /*
>> +             * As SME is not yet supported by KVM, ignore reading SMIDR_EL1
>> +             */
>> +             break;
>> +        case GMID_EL1_IDX:
>> +            /*
>> +             * GMID_EL1 is not in KVM's sys_reg_descs[] and guest reads
>> +             * the raw host value. But as it is an EL1 register, we cannot
>> +             * read it directly. This is a genuine miss when MTE is
>> +             * enabled for the guest.
>> +             */
>> +            break;
>> +        default:
>> +            ret = read_sys_reg64(fd, reg, idregs_sysreg_to_kvm_reg(sysreg));
>> +            if (ret) {
>> +                if (errno == ENOENT) {
>> +                    warn_report("%s error reading value of host %s register "
>> +                                    "(ENOENT)", __func__, sysregdesc->name);
> trace point
Noted.

Warm Regards,
Khushit
>> +                } 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);
> Thanks
> 
> Eric