Re: [PATCH v3 07/10] arm/kvm: write back modified ID regs to KVM

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

On 4/14/25 6:38 PM, Cornelia Huck wrote:
> From: Eric Auger <eric.auger@redhat.com>
>
> We want to give a chance to override the value of host ID regs.
> In a previous patch we made sure all their values could be fetched
> through kvm_get_one_reg() calls before their modification. After
> their potential modification we need to make sure we write back
nit: write their value back?

Eric
> the values through kvm_set_one_reg() calls.
>
> Make sure the cpreg_list is modified with updated values and
> transfer those values back to kvm.
>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> ---
>  target/arm/kvm.c        | 44 ++++++++++++++++++++++++++++++++++++++++-
>  target/arm/trace-events |  1 +
>  2 files changed, 44 insertions(+), 1 deletion(-)
>
> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
> index b07d5f16db50..9e4cca1705c8 100644
> --- a/target/arm/kvm.c
> +++ b/target/arm/kvm.c
> @@ -1083,6 +1083,39 @@ void kvm_arm_cpu_post_load(ARMCPU *cpu)
>      }
>  }
>  
> +static void kvm_arm_writable_idregs_to_cpreg_list(ARMCPU *cpu)
> +{
> +    if (!cpu->writable_map) {
> +        return;
> +    }
> +    for (int i = 0; i < NR_ID_REGS; i++) {
> +        uint64_t writable_mask = cpu->writable_map->regs[i];
> +        uint64_t *cpreg;
> +
> +        if (writable_mask) {
> +            uint64_t previous, new;
> +            int idx = kvm_idx_to_idregs_idx(i);
> +            ARM64SysReg *sysregdesc;
> +            uint32_t sysreg;
> +
> +            if (idx == -1) {
> +                /* sysreg writable, but we don't know it */
> +                continue;
> +            }
> +            sysregdesc = &arm64_id_regs[idx];
> +            sysreg = sysregdesc->sysreg;
> +            cpreg = kvm_arm_get_cpreg_ptr(cpu, idregs_sysreg_to_kvm_reg(sysreg));
> +            previous = *cpreg;
> +            new = cpu->isar.idregs[idx];
> +            if (previous != new) {
> +                *cpreg = new;
> +                trace_kvm_arm_writable_idregs_to_cpreg_list(sysregdesc->name,
> +                                                            previous, new);
> +            }
> +        }
> +    }
> +}
> +
>  void kvm_arm_reset_vcpu(ARMCPU *cpu)
>  {
>      int ret;
> @@ -2050,7 +2083,16 @@ int kvm_arch_init_vcpu(CPUState *cs)
>      }
>      cpu->mp_affinity = mpidr & ARM64_AFFINITY_MASK;
>  
> -    return kvm_arm_init_cpreg_list(cpu);
> +    ret = kvm_arm_init_cpreg_list(cpu);
> +    if (ret) {
> +        return ret;
> +    }
> +    /* overwrite writable ID regs with their updated property values */
> +    kvm_arm_writable_idregs_to_cpreg_list(cpu);
> +
> +    write_list_to_kvmstate(cpu, 3);
> +
> +    return 0;
>  }
>  
>  int kvm_arch_destroy_vcpu(CPUState *cs)
> diff --git a/target/arm/trace-events b/target/arm/trace-events
> index 17e52c0705f2..955149ee1ac4 100644
> --- a/target/arm/trace-events
> +++ b/target/arm/trace-events
> @@ -14,3 +14,4 @@ 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
> +kvm_arm_writable_idregs_to_cpreg_list(const char *name, uint64_t previous, uint64_t new) "%s overwrite default 0x%"PRIx64" with 0x%"PRIx64