Re: [RFC 1/3] target/arm/cpu: Add new CPU property for KVM regs to hide

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

On 10/8/25 3:43 PM, Cornelia Huck wrote:
> On Thu, Sep 11 2025, Eric Auger <eric.auger@redhat.com> wrote:
>
>> New kernels sometimes expose new registers in an unconditionnal
>>  manner.  This situation breaks backward migration as qemu notices
>> there are more registers to store on guest than supported in the
>> destination kerenl. This leads to a "failed to load
>> cpu:cpreg_vmstate_array_len" error.
>>
>> A good example is the introduction of KVM_REG_ARM_VENDOR_HYP_BMAP_2
>> pseudo FW register in v6.16 by commit C0000e58c74e (“KVM: arm64:
>> Introduce KVM_REG_ARM_VENDOR_HYP_BMAP_2”). Trying to do backward
>> migration from a host kernel which features the commit to a destination
>> host that doesn't fail.
>>
>> Currently QEMU is not using that feature so ignoring this latter
>> is not a problem. An easy way to fix the migration issue is to teach
>> qemu we don't care about that register and we can simply ignore it,
>> including its state migration.
>>
>> This patch introduces a CPU property, under the form of an array of
>> reg indices which indicates which registers can be ignored.
>>
>> The goal then is to set this property in machine type compats such
>> as:
>> static GlobalProperty arm_virt_kernel_compat_10_1[] = {
>>     /* KVM_REG_ARM_VENDOR_HYP_BMAP_2 */
>>     { TYPE_ARM_CPU, "kvm-hidden-regs", "0x6030000000160003" },
>> }
>>
>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>> ---
>>  target/arm/cpu.h        |  4 ++++
>>  target/arm/kvm.c        | 36 ++++++++++++++++++++++++++++++++++--
>>  target/arm/trace-events |  2 ++
>>  3 files changed, 40 insertions(+), 2 deletions(-)
>>
> (...)
>
>> +/**
>> + * kvm_vcpu_compat_hidden_reg:
>> + * @cpu: ARMCPU
>> + * @regidx: index of the register to check
>> + *
>> + * Depending on the CPU compat returns true if @regidx must be
>> + * ignored during sync & migration
> Maybe add some more explanation, either here or at the kvm_hidden_regs[]
> def? So that people do not need to look at the introducing commit :)
>
> "This is intended for when we know that we do not use the register to be
> ignored, and want to keep compatibility."

Added here and also in next patch, in cpu.h

Thanks!

Eric
>
>> + */
>> +static inline bool
>> +kvm_vcpu_compat_hidden_reg(ARMCPU *cpu, uint64_t regidx)
>> +{
>> +    for (int i = 0; i < cpu->nr_kvm_hidden_regs; i++) {
>> +        if (cpu->kvm_hidden_regs[i] == regidx) {
>> +            trace_kvm_vcpu_compat_hidden_reg(regidx);
>> +            return true;
>> +        }
>> +    }
>> +    return false;
>> +}