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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_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_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 10/9/2025 10:19 AM, Daniel P. Berrangé wrote: > On Thu, Oct 09, 2025 at 10:11:17AM -0400, Steven Sistare wrote: >> On 10/8/2025 6:22 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 9/30/2025 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()) { >>> >>> OK, will do, clearing kvm_allowed is a good idea. >>> >>> Currently there are two qmp commands "query-balloon" and "query-cpu-definitions" >>> causing SIGSEGV after CPR-transfer, clearing kvm_allowed helps for both. >>> But clearing both kvm_allowed and kvm_state cause SIGSEGV on "query-cpu-definitions". >>> >>> I'll send a patch to clearing only kvm_allowed if you are fine with it. >> >> I still don't love the idea. kvm_state is no longer valid. >> >> It sounds like "query-cpu-definitions" is missing a check for kvm_enabled(). > > This patch / bug feels like it is side-stepping a general conceptual > question. After cpr-transfer is complete what QMP commands are still > valid to call in general ? This thread mentions two which have caused > bugs, but beyond that it feels like a large subset of QMP comamnds > are conceptually invalid to use. Agreed. I don't see why these commands should be issued to the dead instance. How about migration status, quit, and nothing else? Half serious. - Steve