{"id":2221531,"url":"http://patchwork.ozlabs.org/api/1.1/patches/2221531/?format=json","web_url":"http://patchwork.ozlabs.org/project/qemu-devel/patch/20260409161042.55281-1-harshpb@linux.ibm.com/","project":{"id":14,"url":"http://patchwork.ozlabs.org/api/1.1/projects/14/?format=json","name":"QEMU Development","link_name":"qemu-devel","list_id":"qemu-devel.nongnu.org","list_email":"qemu-devel@nongnu.org","web_url":"","scm_url":"","webscm_url":""},"msgid":"<20260409161042.55281-1-harshpb@linux.ibm.com>","date":"2026-04-09T16:10:42","name":"[for,11.0-rc3] accel/kvm: Fix BQL lock imbalance in kvm_cpu_exec","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"fab9819f1b42e65ca43977cceba6475c8421eacb","submitter":{"id":85411,"url":"http://patchwork.ozlabs.org/api/1.1/people/85411/?format=json","name":"Harsh Prateek Bora","email":"harshpb@linux.ibm.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/qemu-devel/patch/20260409161042.55281-1-harshpb@linux.ibm.com/mbox/","series":[{"id":499325,"url":"http://patchwork.ozlabs.org/api/1.1/series/499325/?format=json","web_url":"http://patchwork.ozlabs.org/project/qemu-devel/list/?series=499325","date":"2026-04-09T16:10:42","name":"[for,11.0-rc3] accel/kvm: Fix BQL lock imbalance in kvm_cpu_exec","version":1,"mbox":"http://patchwork.ozlabs.org/series/499325/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2221531/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2221531/checks/","tags":{},"headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=ibm.com header.i=@ibm.com header.a=rsa-sha256\n header.s=pp1 header.b=VrKFslVR;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=cc\n :content-transfer-encoding:date:from:message-id:mime-version\n :subject:to; s=pp1; bh=pMxwQeZ4ev4mCAV9Ck0oKLFAv7ZIKhndqQKrvJsvl\n 6Y=; b=VrKFslVRPBWpbRQWJRLSZ+d/eEvAWKaSM7hMBkaV+k7HfJDNmqD63p7cU\n hpLW9ck+N5wO5ERIPSipLw/KTloxQuTOTWWKapRcSkQKzoa++8Tmv7ooMK1UIc6w\n dFO/8sFdEdgtiRODALnBw18K1O3DpxAd1m+1CbUR1oaPYoTWUjo4QNJhTOxyvstQ\n /4wZBToeVpflL6mJW5LCMkQGZIPGNmjWyg+Z191TOZdDWMiGW+eWoXBLSTzBFJ2x\n Th/iTtPVuq6IaVU71V7NwJXZn8NovigElWc+dmrnIUb22Or1y4UPX4Sl0JpR2JRM\n 6VNR8eVTfVaKff2mbGrPbNswtWKTg==","From":"Harsh Prateek Bora ","To":"qemu-devel@nongnu.org, qemu-ppc@nongnu.org","Cc":"anisinha@redhat.com, pbonzini@redhat.com, npiggin@gmail.com,\n misanjum@linux.ibm.com, gautam@linux.ibm.com, peter.maydell@linaro.org","Subject":"[PATCH for 11.0-rc3] accel/kvm: Fix BQL lock imbalance in\n kvm_cpu_exec","Date":"Thu, 9 Apr 2026 21:40:42 +0530","Message-ID":"<20260409161042.55281-1-harshpb@linux.ibm.com>","X-Mailer":"git-send-email 2.52.0","MIME-Version":"1.0","Content-Transfer-Encoding":"8bit","X-TM-AS-GCONF":"00","X-Proofpoint-Reinject":"loops=2 maxloops=12","X-Proofpoint-Spam-Details-Enc":"AW1haW4tMjYwNDA5MDE0NSBTYWx0ZWRfX14lecYEXauIs\n VGw3irFbeq8+f045DWTY80qEW0Kptvd+8cwdgkUmr50MkIXWeqjTe6w0zBhT56aEY6HKnf632qu\n R7tTLpDmx0nZtgmqeH5BpnzCqlNLP+zyjHhgsGId6dYZobrOZWRhRrf6lF6Ikk9UguoN7FVXknz\n aKqHCGSvySvnoTiZtEs/83Rfh27/uchUHB676KkjE2J326O16WwiH2nswytVzGjM1/aPJJMWdu8\n cEzBDTBXu3YlAiTKgEbvrzJNB/pavgK0C5DaZifoiz1C10F7Wqa4mYYWcErtLFvAg2CD9z3SRCb\n sDbRXKUPjh451gxoeQcHd3e79mdkM7aUCfkccmMQD/IjeqzMwMZC5vwfX45ll06ZmnR7i52QHK/\n c0dwCfLYqmpnVQoDTBlRKpl/hJaIu5csiW1iGojgeXcksjwhU9OTuWoYKVb22BWR0E0Ehm0uqkb\n X+XO8Q0i63AEB2v3t/g==","X-Proofpoint-GUID":"mB8bpi-nAlk1Q2GS0Bo8tnBoTzVVH4Ph","X-Authority-Analysis":"v=2.4 cv=a/wAM0SF c=1 sm=1 tr=0 ts=69d7cf8e cx=c_pps\n a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17\n a=A5OVakUREuEA:10 a=f7IdgyKtn90A:10 a=VkNPw1HP01LnGYTKEx00:22\n a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=VnNF1IyMAAAA:8\n a=J3LV9hoIG5fqms1vbrYA:9 a=O8hF6Hzn-FEA:10","X-Proofpoint-ORIG-GUID":"S9AEwKEdiqit0oF60W46EMXD0qU6EZUR","X-Proofpoint-Virus-Version":"vendor=baseguard\n engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.51,FMLib:17.12.100.49\n definitions=2026-04-09_04,2026-04-09_02,2025-10-01_01","X-Proofpoint-Spam-Details":"rule=outbound_notspam policy=outbound score=0\n bulkscore=0 clxscore=1015 spamscore=0 impostorscore=0 priorityscore=1501\n phishscore=0 lowpriorityscore=0 adultscore=0 malwarescore=0 suspectscore=0\n classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0\n reason=mlx scancount=1 engine=8.22.0-2604010000 definitions=main-2604090145","Received-SPF":"pass client-ip=148.163.158.5;\n envelope-from=harshpb@linux.ibm.com;\n helo=mx0b-001b2d01.pphosted.com","X-Spam_score_int":"-26","X-Spam_score":"-2.7","X-Spam_bar":"--","X-Spam_report":"(-2.7 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1,\n DKIM_VALID=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7,\n RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001,\n RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001,\n 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":"qemu development ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org","Sender":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org"},"content":"When kvm_cpu_exec() returns EXCP_HLT due to kvm_arch_process_async_events()\nreturning true, it was returning before releasing the BQL (Big QEMU Lock).\nThis caused a lock imbalance where the vCPU thread would loop back to\nkvm_cpu_exec() while still holding the BQL, leading to deadlocks.\n\nThe issue manifests as boot hangs on PowerPC pseries machines with multiple\nvCPUs, where secondary vCPUs with start-powered-off=true remain halted and\nrepeatedly call kvm_cpu_exec() which returns EXCP_HLT. Each iteration held\nthe BQL, preventing other operations from proceeding.\n\nThe fix has two parts:\n\n1. In kvm_cpu_exec() (kvm-all.c):\n Release the BQL before returning EXCP_HLT in the early return path,\n matching the behavior of the normal execution path where bql_unlock()\n is called before entering the main KVM execution loop.\n\n2. In kvm_vcpu_thread_fn() (kvm-accel-ops.c):\n Re-acquire the BQL after kvm_cpu_exec() returns EXCP_HLT, since the\n loop expects to hold the BQL when calling kvm_cpu_exec() again.\n\nThis ensures proper BQL lock/unlock pairing:\n- kvm_vcpu_thread_fn() holds BQL before calling kvm_cpu_exec()\n- kvm_cpu_exec() releases BQL before returning (for EXCP_HLT)\n- kvm_vcpu_thread_fn() re-acquires BQL if EXCP_HLT was returned\n- Next iteration has BQL held as expected\n\nThis is a regression introduced by commit 98884e0cc1 (\"accel/kvm: add\nchanges required to support KVM VM file descriptor change\") which\nrefactored kvm_irqchip_create() and changed the initialization timing,\nexposing this lock imbalance issue.\n\nFixes: 98884e0cc1 (\"accel/kvm: add changes required to support KVM VM file descriptor change\")\nReported-by: Misbah Anjum N \nReported-by: Gautam Menghani \nSigned-off-by: Harsh Prateek Bora \n---\n accel/kvm/kvm-accel-ops.c | 4 ++++\n accel/kvm/kvm-all.c | 1 +\n 2 files changed, 5 insertions(+)","diff":"diff --git a/accel/kvm/kvm-accel-ops.c b/accel/kvm/kvm-accel-ops.c\nindex 6d9140e549..d684fd0840 100644\n--- a/accel/kvm/kvm-accel-ops.c\n+++ b/accel/kvm/kvm-accel-ops.c\n@@ -52,6 +52,10 @@ static void *kvm_vcpu_thread_fn(void *arg)\n \n if (cpu_can_run(cpu)) {\n r = kvm_cpu_exec(cpu);\n+ if (r == EXCP_HLT) {\n+ /* kvm_cpu_exec() released BQL, re-acquire for next iteration */\n+ bql_lock();\n+ }\n if (r == EXCP_DEBUG) {\n cpu_handle_guest_debug(cpu);\n }\ndiff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c\nindex 774499d34f..00b8018664 100644\n--- a/accel/kvm/kvm-all.c\n+++ b/accel/kvm/kvm-all.c\n@@ -3439,6 +3439,7 @@ int kvm_cpu_exec(CPUState *cpu)\n trace_kvm_cpu_exec();\n \n if (kvm_arch_process_async_events(cpu)) {\n+ bql_unlock();\n return EXCP_HLT;\n }\n \n","prefixes":["for","11.0-rc3"]}