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

[Cornelia Huck <cohuck@redhat.com>]

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."

> + */
> +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;
> +}