Hi Connie On 12/22/25 2:17 PM, Eric Auger wrote: > > > On 11/25/25 6:04 PM, Cornelia Huck wrote: >> On Tue, Nov 25 2025, Eric Auger <[email protected]> wrote: >> >>> More recent kernels sometimes expose new registers in an >>> unconditionnal manner. This situation breaks backward migration >>> as qemu notices there are more registers in the input stream >>> than supported on the destination host. 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 that features the commit to a destination >>> host that doesn't, fail with above error. >>> >>> 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 >>> when syncing its state during migration. >>> >>> This patch introduces an array of such hidden registers. Soon it will >>> be settable through an array property. >>> >>> If hidden, the register is moved out of the array of cpreg which is >>> built in kvm_arm_init_cpreg_list(). That way their state won't be >>> synced. >> I'm wondering whether the patch description should also mention non-KVM >> cases (e.g. the bogus reg that was exposed). It might also make sense to >> merge patch 5 here? > > Actually the AArch32 DBGDTRTX is not "hidden" on destination but rather > marked as safe to ignore in the incoming stream (so the other category). > So I don't have any example of TCG usage for this category. > > wrt potiential merge I agree they could. 2nd patch is an enforcement of > non access so I prefer to keep them separate but will put them > contiguously in the series. Sorry I mixed up with 6/11. I will squash 5/11 in this patch.
Thanks! Eric> > Eric >> >>> Signed-off-by: Eric Auger <[email protected]> >> Otherwise, LGTM. >> >
