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X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 9/30/2025 2:00 AM, Duan, Zhenzhong wrote: >> -----Original Message----- >> From: Steven Sistare >> Subject: Re: [PATCH v2 6/6] accel/kvm: Fix SIGSEGV when execute >> "query-balloon" after CPR transfer >> >> On 9/28/2025 4:54 AM, Zhenzhong Duan wrote: >>> After CPR transfer, source QEMU closes kvm fd and sets kvm_state to >> NULL, >>> "query-balloon" will check kvm_state->sync_mmu and trigger NULL pointer >>> reference. >>> >>> We don't need to NULL kvm_state as all states in kvm_state aren't released >>> actually. Just closing kvm fd is enough so we could still query states >>> through "query_*" qmp command. >> >> IMO this does not make sense. Much of the state in kvm_state was derived >>from ioctl's on the descriptors, and closing them invalidates it. Asking >> historical questions about what used to be makes no sense. > > You also have your valid point. > >> >> Clearing kvm_state and setting kvm_allowed=false would be a safer fix. Setting kvm_allowed=false causes kvm_enabled() to return false which should prevent kvm_state from being dereferenced anywhere: #define kvm_enabled() (kvm_allowed) Eg for the balloon: static bool have_balloon(Error **errp) { if (kvm_enabled() && !kvm_has_sync_mmu()) { - Steve > But clearing kvm_state will make some qmp commands which dereferencing > kvm_state trigger SIGSEGV. We can expect failure on those qmp, but SIGSEGV > is worse. > > E.g., {"execute": "query-cpu-definitions"}, below error print with v2 but SIGSEGV with v1: > > KVM get MSR (index=0x10a) feature failed, Bad file descriptor > > > I also see inconsistence on "query-balloon" result with v1 patch, before cpr-transfer: > > {"error": {"class": "DeviceNotActive", "desc": "No balloon device has been activated"}} > > after transfer: > > {"error": {"class": "KVMMissingCap", "desc": "Using KVM without synchronous MMU, balloon unavailable"}} > > It's confusing there is no synchronous MMU support but we do have it. > > Thanks > Zhenzhong