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Fri, 01 Aug 2025 03:26:28 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHvd6lCXn+4qds0Gw+4p/7QI/y/eNFrTD6/N6M/IMbJOMnJ0ZFaIJ25+udp+WuofX3EJpuZqw== X-Received: by 2002:a05:6000:1a8d:b0:3b7:644f:9ca7 with SMTP id ffacd0b85a97d-3b794fd7c3cmr8202948f8f.25.1754043988075; Fri, 01 Aug 2025 03:26:28 -0700 (PDT) Received: from [192.168.10.81] ([176.206.122.191]) by smtp.googlemail.com with ESMTPSA id ffacd0b85a97d-3b79c3b9160sm5455002f8f.21.2025.08.01.03.26.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 01 Aug 2025 03:26:27 -0700 (PDT) Message-ID: <8edc80d5-49a0-4e4d-82c4-e4a18eb78304@redhat.com> Date: Fri, 1 Aug 2025 12:26:26 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 6/6] kvm: i386: irqchip: take BQL only if there is an interrupt To: Igor Mammedov , qemu-devel@nongnu.org Cc: peterx@redhat.com, mst@redhat.com, mtosatti@redhat.com, kraxel@redhat.com, peter.maydell@linaro.org References: <20250730123934.1787379-1-imammedo@redhat.com> <20250730123934.1787379-7-imammedo@redhat.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=pbonzini@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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-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 The patch is not wrong but complicates things more than it should. Also, as we do more of these tricks it may be worth adding wrapper APIs for interrupt_request access, but that needs to be done tree-wide so you can do it separately. On 7/30/25 14:39, Igor Mammedov wrote: > if (cpu->interrupt_request & (CPU_INTERRUPT_INIT | CPU_INTERRUPT_TPR)) { > + if (!kvm_pic_in_kernel()) { > + bql_lock(); > + release_bql = true; > + } This bql_lock() is not needed, all the writes in the "if" are local to the current CPU. When the outer bql_lock() was added, cpu_interrupt() was not thread-safe at all, and taking the lock was needed in order to read cpu->interrupt_request. But now it is ok to read outside the lock, which you can use to simplify this patch a lot. > if ((cpu->interrupt_request & CPU_INTERRUPT_INIT) && > !(env->hflags & HF_SMM_MASK)) { > cpu->exit_request = 1; A patch that changes all these accesses to qatomic_set(&cpu->exit_request, 1), tree-wide, would be nice. > + if (cpu->interrupt_request & CPU_INTERRUPT_HARD) { This should be qatomic_read(&cpu->interrupt_request). Not a blocker for now, but this is where I would suggest adding a wrapper like cpu_test_interrupt(cpu, CPU_INTERRUPT_HARD). > + if (!release_bql) { > + bql_lock(); > + release_bql = true; > + } With the above simplification, this can be done unconditionally. > + /* Try to inject an interrupt if the guest can accept it */ > + if (run->ready_for_interrupt_injection && > + (cpu->interrupt_request & CPU_INTERRUPT_HARD) && > + (env->eflags & IF_MASK)) { > + int irq; > + > + cpu->interrupt_request &= ~CPU_INTERRUPT_HARD; Reads and writes to cpu->interrupt_request still take the BQL, which is consistent with include/hw/core/cpu.h, so yeah here the bql_lock() is needed. Like above, writing it's a data race with readers outside the BQL, so qatomic_read()/qatomic_set() would be needed to respect the C standard. Even better could be to add a function cpu_reset_interrupt_locked() that does assert(bql_locked()); qatomic_set(&cpu->interrupt_request, cpu->interrupt_request & ~mask); But neither of these wrappers (which should be applied tree-wide) are an absolute necessity for this series. > @@ -5531,7 +5540,14 @@ void kvm_arch_pre_run(CPUState *cpu, struct kvm_run *run) > > DPRINTF("setting tpr\n"); > run->cr8 = cpu_get_apic_tpr(x86_cpu->apic_state); > + /* > + * make sure that request_interrupt_window/cr8 are set > + * before KVM_RUN might read them > + */ > + smp_mb(); This is not needed, ->cr8 is only read for the same CPU (in kvm_arch_vcpu_ioctl_run). > + } > > + if (release_bql) { > bql_unlock(); > } And since release_bql is not needed anymore, the bql_unlock() can be left where it was. Paolo > }