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([2a01:e0a:f0e:9070:527b:9dff:feef:3874]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-429952cb55asm19518873f8f.17.2025.10.28.03.05.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 28 Oct 2025 03:05:53 -0700 (PDT) Message-ID: <3c963761-8587-4905-8c8b-60dda381262f@redhat.com> Date: Tue, 28 Oct 2025 11:05:52 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RESEND PATCH 0/7] Mitigation of "failed to load cpu:cpreg_vmstate_array_len" migration failures Content-Language: en-US To: eric.auger.pro@gmail.com, qemu-devel@nongnu.org, qemu-arm@nongnu.org, peter.maydell@linaro.org, cohuck@redhat.com, maz@kernel.org, oliver.upton@linux.dev, sebott@redhat.com, gshan@redhat.com, ddutile@redhat.com, peterx@redhat.com, philmd@linaro.org, pbonzini@redhat.com References: <20251016140039.250111-1-eric.auger@redhat.com> From: Eric Auger In-Reply-To: <20251016140039.250111-1-eric.auger@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=eric.auger@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, T_SPF_TEMPERROR=0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: eric.auger@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 10/16/25 3:59 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 Gentle ping. Any comments on the approach? Thanks 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(-) >