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

[Sebastian Ott <sebott@redhat.com>]

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On Thu, 11 Sep 2025, Eric Auger 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" },
> }

One thing worth noting - once this series lands:
https://lore.kernel.org/qemu-devel/20250801074730.28329-1-shameerkolothum@gmail.com/
we might need to add a bit more logic here. Either using the kvm
interfaces (only ignore KVM_REG_ARM_VENDOR_HYP_BMAP_2 when the register
value is 0) or qemu knowledge (only ignore KVM_REG_ARM_VENDOR_HYP_BMAP_2
when the impl-cpu property is not used).

Sebastian