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[34.168.104.7]) by smtp.gmail.com with ESMTPSA id d62-20020a623641000000b0056c08c87196sm9173979pfa.48.2022.11.15.12.16.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 15 Nov 2022 12:16:07 -0800 (PST) Date: Tue, 15 Nov 2022 20:16:04 +0000 From: Sean Christopherson To: "Huang, Kai" Cc: "farman@linux.ibm.com" , "frankja@linux.ibm.com" , "mjrosato@linux.ibm.com" , "vkuznets@redhat.com" , "chenhuacai@kernel.org" , "aou@eecs.berkeley.edu" , "palmer@dabbelt.com" , "paul.walmsley@sifive.com" , "maz@kernel.org" , "anup@brainfault.org" , "imbrenda@linux.ibm.com" , "pbonzini@redhat.com" , "borntraeger@linux.ibm.com" , "aleksandar.qemu.devel@gmail.com" , "kvm@vger.kernel.org" , "atishp@atishpatra.org" , "farosas@linux.ibm.com" , "david@redhat.com" , "Yao, Yuan" , "mpe@ellerman.id.au" , "alexandru.elisei@arm.com" , "linux-s390@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "tglx@linutronix.de" , "Yamahata, Isaku" , "kvmarm@lists.linux.dev" , "james.morse@arm.com" , "kvm-riscv@lists.infradead.org" , "suzuki.poulose@arm.com" , "linuxppc-dev@lists.ozlabs.org" , "linux-arm-kernel@lists.infradead.org" , "linux-mips@vger.kernel.org" , "Gao, Chao" , "oliver.upton@linux.dev" , "kvmarm@lists.cs.columbia.edu" , "linux-riscv@lists.infradead.org" Subject: Re: [PATCH 38/44] KVM: Disable CPU hotplug during hardware enabling Message-ID: References: <20221102231911.3107438-1-seanjc@google.com> <20221102231911.3107438-39-seanjc@google.com> <88e920944de70e7d69a98f74005b49c59b5aaa3b.camel@intel.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20221115_121613_378124_4FD2A3A9 X-CRM114-Status: GOOD ( 25.41 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Nov 10, 2022, Huang, Kai wrote: > On Thu, 2022-11-10 at 01:33 +0000, Huang, Kai wrote: > > > @@ -9283,7 +9283,13 @@ static int > > > kvm_x86_check_processor_compatibility(struct kvm_x86_init_ops *ops) > > > =A0 int cpu =3D smp_processor_id(); > > > =A0 struct cpuinfo_x86 *c =3D &cpu_data(cpu); > > > =A0 > > > - WARN_ON(!irqs_disabled()); > > > + /* > > > + * Compatibility checks are done when loading KVM and when enabling > > > + * hardware, e.g. during CPU hotplug, to ensure all online CPUs are > > > + * compatible, i.e. KVM should never perform a compatibility check > > > on > > > + * an offline CPU. > > > + */ > > > + WARN_ON(!irqs_disabled() && cpu_active(cpu)); > > > =A0 > > = > > Also, the logic of: > > = > > !irqs_disabled() && cpu_active(cpu) > > = > > is quite weird. > > = > > The original "WARN(!irqs_disabled())" is reasonable because in STARTING > > section > > the IRQ is indeed disabled. > > = > > But this doesn't make sense anymore after we move to ONLINE section, in= which > > IRQ has already been enabled (see start_secondary()).=A0 IIUC the WARN_= ON() > > doesn't get exploded is purely because there's an additional cpu_active= (cpu) > > check. > > = > > So, a more reasonable check should be something like: > > = > > WARN_ON(irqs_disabled() || cpu_active(cpu) || !cpu_online(cpu)); > > = > > Or we can simply do: > > = > > WARN_ON(!cpu_online(cpu) || cpu_active(cpu)); > > = > > (because I don't know whether it's possible IRQ can somehow get disable= d in > > ONLINE section). > > = > > Btw above is purely based on code analysis, but I haven't done any test. > = > Hmm.. I wasn't thinking thoroughly. I forgot CPU compatibility check also > happens on all online cpus when loading KVM. For this case, IRQ is disab= led and > cpu_active() is true. For the hotplug case, IRQ is enabled but cpu_acti= ve() is > false. Actually, you're right (and wrong). You're right in that the WARN is flawe= d. And the reason for that is because you're wrong about the hotplug case. In thi= s version of things, the compatibility checks are routed through hardware enabling, i= .e. this flow is used only when loading KVM. This helper should only be called via = SMP function call, which means that IRQs should always be disabled. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel