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Fri, 06 Nov 2026 09:44:13 -0800 (PST) Received: from [192.168.0.20] ([212.21.133.107]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4b76d83e539sm8359915e9.1.2026.11.06.09.44.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 06 Nov 2026 09:44:12 -0800 (PST) Message-ID: Date: Fri, 6 Nov 2026 19:44:11 +0200 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 2/9] stop_machine: Add NMI-based execution path To: "Chang S. Bae" , linux-kernel@vger.kernel.org Cc: x86@kernel.org, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, peterz@infradead.org, david.kaplan@amd.com References: <20261015211426.49114-1-chang.seok.bae@intel.com> <20261015211426.49114-3-chang.seok.bae@intel.com> From: Nikolay Borisov Content-Language: en-US Autocrypt: addr=nik.borisov@suse.com; keydata= xsFNBGcrpvIBEAD5cAR5+qu30GnmPrK9veWX5RVzzbgtkk9C/EESHy9Yz0+HWgCVRoNyRQsZ 7DW7vE1KhioDLXjDmeu8/0A8u5nFMqv6d1Gt1lb7XzSAYw7uSWXLPEjFBtz9+fBJJLgbYU7G OpTKy6gRr6GaItZze+r04PGWjeyVUuHZuncTO7B2huxcwIk9tFtRX21gVSOOC96HcxSVVA7X N/LLM2EOL7kg4/yDWEhAdLQDChswhmdpHkp5g6ytj9TM8bNlq9I41hl/3cBEeAkxtb/eS5YR 88LBb/2FkcGnhxkGJPNB+4Siku7K8Mk2Y6elnkOctJcDvk29DajYbQnnW4nhfelZuLNupb1O M0912EvzOVI0dIVgR+xtosp66bYTOpX4Xb0fylED9kYGiuEAeoQZaDQ2eICDcHPiaLzh+6cc pkVTB0sXkWHUsPamtPum6/PgWLE9vGI5s+FaqBaqBYDKyvtJfLK4BdZng0Uc3ijycPs3bpbQ bOnK9LD8TYmYaeTenoNILQ7Ut54CCEXkP446skUMKrEo/HabvkykyWqWiIE/UlAYAx9+Ckho TT1d2QsmsAiYYWwjU8igXBecIbC0uRtF/cTfelNGrQwbICUT6kJjcOTpQDaVyIgRSlUMrlNZ XPVEQ6Zq3/aENA8ObhFxE5PLJPizJH6SC89BMKF3zg6SKx0qzQARAQABzSZOaWtvbGF5IEJv cmlzb3YgPG5pay5ib3Jpc292QHN1c2UuY29tPsLBkQQTAQoAOxYhBDuWB8EJLBUZCPjT3SRn XZEnyhfsBQJnK6byAhsDBQsJCAcCAiICBhUKCQgLAgQWAgMBAh4HAheAAAoJECRnXZEnyhfs XbIQAJxuUnelGdXbSbtovBNm+HF3LtT0XnZ0+DoR0DemUGuA1bZAlaOXGr5mvVbTgaoGUQIJ 3Ejx3UBEG7ZSJcfJobB34w1qHEDO0pN9orGIFT9Bic3lqhawD2r85QMcWwjsZH5FhyRx7P2o DTuUClLMO95GuHYQngBF2rHHl8QMJPVKsR18w4IWAhALpEApxa3luyV7pAAqKllfCNt7tmed uKmclf/Sz6qoP75CvEtRbfAOqYgG1Uk9A62C51iAPe35neMre3WGLsdgyMj4/15jPYi+tOUX Tc7AAWgc95LXyPJo8069MOU73htZmgH4OYy+S7f+ArXD7h8lTLT1niff2bCPi6eiAQq6b5CJ Ka4/27IiZo8tm1XjLYmoBmaCovqx5y5Xt2koibIWG3ZGD2I+qRwZ0UohKRH6kKVHGcrmCv0J YO8yIprxgoYmA7gq21BpTqw3D4+8xujn/6LgndLKmGESM1FuY3ymXgj5983eqaxicKpT9iq8 /a1j31tms4azR7+6Dt8H4SagfN6VbJ0luPzobrrNFxUgpjR4ZyQQ++G7oSRdwjfIh1wuCF6/ mDUNcb6/kA0JS9otiC3omfht47yQnvod+MxFk1lTNUu3hePJUwg1vT1te3vO5oln8lkUo9BU knlYpQ7QA2rDEKs+YWqUstr4pDtHzwQ6mo0rqP+zzsFNBGcrpvIBEADGYTFkNVttZkt6e7yA LNkv3Q39zQCt8qe7qkPdlj3CqygVXfw+h7GlcT9fuc4kd7YxFys4/Wd9icj9ZatGMwffONmi LnUotIq2N7+xvc4Xu76wv+QJpiuGEfCDB+VdZOmOzUPlmMkcJc/EDSH4qGogIYRu72uweKEq VfBI43PZIGpGJ7TjS3THX5WVI2YNSmuwqxnQF/iVqDtD2N72ObkBwIf9GnrOgxEyJ/SQq2R0 g7hd6IYk7SOKt1a8ZGCN6hXXKzmM6gHRC8fyWeTqJcK4BKSdX8PzEuYmAJjSfx4w6DoxdK5/ 9sVrNzaVgDHS0ThH/5kNkZ65KNR7K2nk45LT5Crjbg7w5/kKDY6/XiXDx7v/BOR/a+Ryo+lM MffN3XSnAex8cmIhNINl5Z8CAvDLUtItLcbDOv7hdXt6DSyb65CdyY8JwOt6CWno1tdjyDEG 5ANwVPYY878IFkOJLRTJuUd5ltybaSWjKIwjYJfIXuoyzE7OL63856MC/Os8PcLfY7vYY2LB cvKH1qOcs+an86DWX17+dkcKD/YLrpzwvRMur5+kTgVfXcC0TAl39N4YtaCKM/3ugAaVS1Mw MrbyGnGqVMqlCpjnpYREzapSk8XxbO2kYRsZQd8J9ei98OSqgPf8xM7NCULd/xaZLJUydql1 JdSREId2C15jut21aQARAQABwsF2BBgBCgAgFiEEO5YHwQksFRkI+NPdJGddkSfKF+wFAmcr pvICGwwACgkQJGddkSfKF+xuuxAA4F9iQc61wvAOAidktv4Rztn4QKy8TAyGN3M8zYf/A5Zx VcGgX4J4MhRUoPQNrzmVlrrtE2KILHxQZx5eQyPgixPXri42oG5ePEXZoLU5GFRYSPjjTYmP ypyTPN7uoWLfw4TxJqWCGRLsjnkwvyN3R4161Dty4Uhzqp1IkNhl3ifTDYEvbnmHaNvlvvna 7+9jjEBDEFYDMuO/CA8UtoVQXjy5gtOhZZkEsptfwQYc+E9U99yxGofDul7xH41VdXGpIhUj 4wjd3IbgaCiHxxj/M9eM99ybu5asvHyMo3EFPkyWxZsBlUN/riFXGspG4sT0cwOUhG2ZnExv XXhOGKs/y3VGhjZeCDWZ+0ZQHPCL3HUebLxW49wwLxvXU6sLNfYnTJxdqn58Aq4sBXW5Un0Q vfbd9VFV/bKFfvUscYk2UKPi9vgn1hY38IfmsnoS8b0uwDq75IBvup9pYFyNyPf5SutxhFfP JDjakbdjBoYDWVoaPbp5KAQ2VQRiR54lir/inyqGX+dwzPX/F4OHfB5RTiAFLJliCxniKFsM d8eHe88jWjm6/ilx4IlLl9/MdVUGjLpBi18X7ejLz3U2quYD8DBAGzCjy49wJ4Di4qQjblb2 pTXoEyM2L6E604NbDu0VDvHg7EXh1WwmijEu28c/hEB6DwtzslLpBSsJV0s1/jE= In-Reply-To: <20261015211426.49114-3-chang.seok.bae@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 16.10.26 г. 0:14 ч., Chang S. Bae wrote: > Currently multi_cpu_stop() executes the stop function in the > MULTI_STOP_RUN state. But NMIs can still preempt the execution. For use > cases requiring stricter isolation from them, provide an option to run > the stop function from NMI context. > > Then, the NMI stop function must be built without instrumentation, as > exceptions may re-enable NMIs on return via IRET. Annotate the NMI > handler entirely with noinstr. > > However, objtool cannot reliably determine whether the indirect call > target is located in a noinstr section, so it may emit false positives. > To avoid this, temporarily lift the instrumentation restriction around > the indirect call site and document the intention. > > The x86 microcode loader is currently the primary user for this. But > other architectures are not expected to use it. Add a build option to > make it opt-in. > > Originally-by: David Kaplan > Suggested-by: Borislav Petkov > Suggested-by: Thomas Gleixner > Signed-off-by: Chang S. Bae > Link: https://lore.kernel.org/lkml/20260129121729.GRaXtP2aeWkQKegxC2@fat_crate.local > Link: https://lore.kernel.org/lkml/87wm0zl8p2.ffs@tglx > --- > V2 -> V3: Address Sashiko review > * Avoid instrumentable functions: > smp_processor_id() -> raw_smp_processor_id() > cpumask_test_and_clear_cpu() -> arch_test_and_clear_bit() > * Remove `done` tag to avoid false setting. Instead, data == NULL as indicator > --- > arch/Kconfig | 3 ++ > include/linux/stop_machine.h | 16 ++++++ > kernel/stop_machine.c | 99 +++++++++++++++++++++++++++++++++++- > 3 files changed, 117 insertions(+), 1 deletion(-) > > diff --git a/arch/Kconfig b/arch/Kconfig > index 45c657772362..afa564261e1c 100644 > --- a/arch/Kconfig > +++ b/arch/Kconfig > @@ -1878,4 +1878,7 @@ config ARCH_HAS_CPU_ATTACK_VECTORS > config HAVE_ARCH_GET_SECUREBOOT > def_bool EFI > > +config STOP_MACHINE_NMI > + bool > + > endmenu > diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h > index 84e7fb627ba4..b53b028296e0 100644 > --- a/include/linux/stop_machine.h > +++ b/include/linux/stop_machine.h > @@ -20,6 +20,12 @@ > */ > typedef int (*cpu_stop_fn_t)(void *arg); > > +/* > + * Stop function variant runnable from NMI context. This makes the > + * noinstr requirement explicit at the type level. > + */ > +typedef int (*cpu_stop_nmisafe_fn_t)(void *arg); > + > #ifdef CONFIG_SMP > > struct cpu_stop_work { > @@ -186,4 +192,14 @@ stop_machine_from_inactive_cpu(cpu_stop_fn_t fn, void *data, > } > > #endif /* CONFIG_SMP || CONFIG_HOTPLUG_CPU */ > + > +#ifdef CONFIG_STOP_MACHINE_NMI > + > +void arch_send_self_nmi(void); > +bool noinstr stop_machine_nmi_handler(void); > + > +#else > +static inline bool stop_machine_nmi_handler(void) { return false; } > +#endif /* CONFIG_STOP_MACHINE_NMI */ > + > #endif /* _LINUX_STOP_MACHINE */ > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c > index 8dcd5be67b85..ad01b7c5266c 100644 > --- a/kernel/stop_machine.c > +++ b/kernel/stop_machine.c > @@ -23,6 +23,7 @@ > #include > #include > #include > +#include > > /* > * Structure to determine completion condition and record errors. May > @@ -175,6 +176,15 @@ struct multi_stop_data { > > enum multi_stop_state state; > atomic_t thread_ack; > + > +#ifdef CONFIG_STOP_MACHINE_NMI > + /* Used in the NMI stop_machine variant */ > + bool use_nmi; > + /* A separate function type to highlight noinstr requirement */ > + cpu_stop_nmisafe_fn_t nmisafe_fn; > + /* cpumask of CPUs on which to raise an NMI */ > + cpumask_var_t nmi_cpus; > +#endif > }; > > static void set_state(struct multi_stop_data *msdata, > @@ -198,6 +208,8 @@ notrace void __weak stop_machine_yield(const struct cpumask *cpumask) > cpu_relax(); > } > > +static int multi_stop_run(struct multi_stop_data *msdata); > + > /* This is the cpu_stop function which stops the CPU. */ > static int multi_cpu_stop(void *data) > { > @@ -236,7 +248,7 @@ static int multi_cpu_stop(void *data) > break; > case MULTI_STOP_RUN: > if (is_active) > - err = msdata->fn(msdata->data); > + err = multi_stop_run(msdata); > break; > default: > break; > @@ -713,3 +725,88 @@ int stop_machine_from_inactive_cpu(cpu_stop_fn_t fn, void *data, > mutex_unlock(&stop_cpus_mutex); > return ret ?: done.ret; > } > + > +#ifdef CONFIG_STOP_MACHINE_NMI > + > +struct nmi_stop { > + struct multi_stop_data *data; > + int ret; > +}; > + > +static DEFINE_PER_CPU(struct nmi_stop, nmi_stop); > + > +/* > + * Instrumentation may trigger nested exceptions such as #INT3, #DB, > + * or #PF. IRET from those would re-enable NMIs, which opposes the goal > + * of this NMI stop-machine facility. > + */ > +bool noinstr stop_machine_nmi_handler(void) > +{ > + struct multi_stop_data *msdata = raw_cpu_read(nmi_stop.data); > + unsigned int cpu = raw_smp_processor_id(); > + int ret; > + > + if (!msdata || !arch_test_and_clear_bit(cpu, cpumask_bits(msdata->nmi_cpus))) > + return false; > + > + /* > + * The indirect call to @nmisafe_fn() is indistinguishable at > + * post-compilation. Temporarily enabling instrumentation avoids > + * objtool false positives. > + */ > + instrumentation_begin(); > + ret = msdata->nmisafe_fn(msdata->data); > + instrumentation_end(); > + > + raw_cpu_write(nmi_stop.ret, ret); > + raw_cpu_write(nmi_stop.data, NULL); > + > + return true; > +} I think this function should be moved over to patch 5 and by having moved/subsumed patch 4, Patch 5 should become patch 3 really so related functionality is introduced in subsequent patches.