Re: [RFC PATCH v4 05/25] target/arm/kvm: read all ID registers from KVM

[Eric Auger <eric.auger@redhat.com>]

Hi Khushit,

On 9/21/26 2:32 PM, Khushit Shah wrote:
> Try to read all ID registers from KVM. KVM may return ENOENT for
> registers that it does not support reading from userspace; ignore
> those registers. Some registers may still require special handling.
>
> This exposes a more complete host feature view to the management
> stack. For migration, both writable and non-writable ID registers
> must match.
>
> Add a tracepoint for ID register reads that fail with ENOENT.
>
> Signed-off-by: Khushit Shah <khushit.shah@nutanix.com>
> ---
>  target/arm/kvm.c        | 29 ++++++++++++++---------------
>  target/arm/trace-events |  1 +
>  2 files changed, 15 insertions(+), 15 deletions(-)
>
> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
> index f8d349a461..d9d0623a2a 100644
> --- a/target/arm/kvm.c
> +++ b/target/arm/kvm.c
> @@ -330,12 +330,12 @@ 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(int fd, ARMHostCPUFeatures *ahcf)
In the parent series I can add a parameter to select if we want only
writable id regs to be read or all id regs
>  {
> @@ -348,19 +348,18 @@ static int get_host_cpu_idregs(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) {
> +                trace_get_host_cpu_idreg_not_found(sysregdesc->name);
> +            } 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;
>  }
> @@ -482,7 +481,7 @@ kvm_arm_get_host_cpu_features(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 */
> +        /* Try to read all the ID registers */
>          err |= get_host_cpu_idregs(fd, ahcf);
>  
>          /*
> diff --git a/target/arm/trace-events b/target/arm/trace-events
> index bbf2e5dfa2..2bb7766f7f 100644
> --- a/target/arm/trace-events
> +++ b/target/arm/trace-events
> @@ -14,6 +14,7 @@ arm_gt_update_irq(int timer, int irqstate) "gt_update_irq: timer %d irqstate %d"
>  # kvm.c
>  kvm_arm_fixup_msi_route(uint64_t iova, uint64_t gpa) "MSI iova = 0x%"PRIx64" is translated into 0x%"PRIx64
>  get_host_cpu_idregs(const char *name, uint64_t value) "scratch vcpu host value for %s is 0x%"PRIx64
> +get_host_cpu_idreg_not_found(const char *name) "host %s register not readable from KVM"
>  kvm_arm_writable_idregs_to_cpreg_list(const char *name, uint64_t previous, uint64_t new) "%s overwrite default 0x%"PRIx64" with 0x%"PRIx64
>  kvm_idreg_write_scratch_vcpu(const char *name, uint64_t oldfv, uint64_t newfv, uint64_t actualfv) "successful dry-run for %s old=0x%"PRIx64" new=0x%"PRIx64" actual=0x%"PRIx64
>  
Eric