Hi, On 10/16/25 3:55 PM, Eric Auger wrote: > When migrating ARM guests accross same machines with different host > kernels we are likely to encounter failures such as: > > "failed to load cpu:cpreg_vmstate_array_len" > > This is due to the fact KVM exposes a different number of registers > to qemu on source and destination. When trying to migrate a bigger > register set to a smaller one, qemu cannot save the CPU state. > > For example, recently we faced such kind of situations with: > - unconditionnal exposure of KVM_REG_ARM_VENDOR_HYP_BMAP_2 FW pseudo > register from v6.16 onwards. Causes backward migration failure. > - removal of unconditionnal exposure of TCR2_EL1, PIRE0_EL1, PIR_EL1 > from v6.13 onwards. Causes forward migration failure. > > This situation is really problematic for distributions which want to > guarantee forward and backward migration of a given machine type > between different releases. > > This small series tries to address that issue by introducing CPU > array properties that list the registers to ignore or to fake according > to the situation. An example is given to illustrate how those props > could be used to apply compats for machine types supposed to "see" the > same register set accross various host kernels. > > The first patch improves the tracing so that we can quickly detect > which registers are unexpected and cause the migration failure. Missing > registers are also traced. Those do not fail migration but their default > value is kept on the destination. > > Then we introduce the infrastructure to handle 'hidden' registers and > 'fake' registers. > > Eric Auger (7): > target/arm/machine: Improve traces on register mismatch during > migration > target/arm/kvm: Introduce the concept of hidden KVM regs > target/arm/kvm: Introduce the concept of enforced/fake registers > kvm-all: Add the capability to blacklist some KVM regs > target/arm/cpu: Implement hide_reg callback() > target/arm/kvm: Expose kvm-hidden-regs and kvm-fake-regs properties > hw/arm/virt: [DO NOT UPSTREAM] Enforce compatibility with older > kernels For some reason the last patch was not properly sent PATCH 7/7] hw/arm/virt: [DO NOT UPSTREAM] Enforce compatibility with older kernels
got "4.7.1 Service unavailable - try again later" So I resent. Sorry for the noise. Eric > > include/hw/core/cpu.h | 2 ++ > target/arm/cpu.h | 42 ++++++++++++++++++++++++ > accel/kvm/kvm-all.c | 12 +++++++ > hw/arm/virt.c | 19 +++++++++++ > target/arm/cpu.c | 12 +++++++ > target/arm/kvm.c | 73 ++++++++++++++++++++++++++++++++++++++++- > target/arm/machine.c | 71 +++++++++++++++++++++++++++++++++++---- > target/arm/trace-events | 11 +++++++ > 8 files changed, 235 insertions(+), 7 deletions(-) >
