From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 00A0AC4332F for ; Tue, 30 Nov 2021 22:31:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243201AbhK3Wez (ORCPT ); Tue, 30 Nov 2021 17:34:55 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54956 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232821AbhK3Wex (ORCPT ); Tue, 30 Nov 2021 17:34:53 -0500 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BCCF4C061574; Tue, 30 Nov 2021 14:31:32 -0800 (PST) From: Thomas Gleixner DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1638311491; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=A0efC9wFH5cPQ7EZaSO684qRGrvGKgOK6GhAO50MOds=; b=v+iHeHgy1NTOZj9j01okmgnGx7ejFbOQZP1oxQAWpmljkH4pTJyZ9mNlx9Gow1RBb1/iIr /RHdfYRSilLwD30p/oIAnVoz8UnhUefxMyhf16480eTu60YLgoWr7ugTgc344/D2Tj1RYl 6fe5nY/hhHlUkdKSaSKcqsuL6plnzG/0Rp/nQ4RYg37+TvSZSax9D763+/ryLvzbSHKWFv /xam0cUjxghQNlT/WzSlf1me7M2SzH9Nr1ecnkcTtt6SIcgniNZjOFLkIzgnzf/A2ctDhJ whqAl21nx9xhPxkxuMiczS2NtBq3i63cjo3THz3C+pJvfApJzlZSoiHB7gCh1g== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1638311491; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=A0efC9wFH5cPQ7EZaSO684qRGrvGKgOK6GhAO50MOds=; b=ZNv872bydOsacHKBu3paDRrkU1mQNSrpW+JmaC6emspigh0VE+CnF4rzvetE58uepldzaR ECTmYs9MIdY3MmCg== To: Steven Rostedt Cc: Nicolas Saenz Julienne , linux-kernel , linux-arm-kernel , rcu@vger.kernel.org, Peter Zijlstra , Mark Rutland , paulmck@kernel.org, mtosatti , frederic , Jonathan Corbet Subject: [PATCH] Documentation: Fill the gaps about entry/noinstr constraints In-Reply-To: <20211130091356.7336e277@gandalf.local.home> References: <8719ad46cc29a2c5d7baac3c35770e5460ab8d5c.camel@redhat.com> <875ys9dacq.ffs@tglx> <20211130091356.7336e277@gandalf.local.home> Date: Tue, 30 Nov 2021 23:31:30 +0100 Message-ID: <878rx5b7i5.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The entry/exit handling for exceptions, interrupts, syscalls and KVM is not really documented except for some comments. Fill the gaps. Reported-by: Nicolas Saenz Julienne Signed-off-by: Thomas Gleixner --- Documentation/core-api/entry.rst | 268 +++++++++++++++++++++++++++++++++++++++ Documentation/core-api/index.rst | 8 + kernel/entry/common.c | 1 3 files changed, 276 insertions(+), 1 deletion(-) --- /dev/null +++ b/Documentation/core-api/entry.rst @@ -0,0 +1,268 @@ +Entry/exit handling for exceptions, interrupts, syscalls and KVM +================================================================ + +For any transition from one execution domain into another the kernel +requires update of various states. The state updates have strict rules +versus ordering. + +The states which need to be updated are: + + * Lockdep + * RCU + * Preemption counter + * Tracing + * Time accounting + +The update order depends on the transition type and is explained below in +the transition type sections. + +Non-instrumentable code - noinstr +--------------------------------- + +Low level transition code cannot be instrumented before RCU is watching and +after RCU went into a non watching state (NOHZ, NOHZ_FULL) as most +instrumentation facilities depend on RCU. + +Aside of that many architectures have to save register state, e.g. debug or +cause registers before another exception of the same type can happen. A +breakpoint in the breakpoint entry code would overwrite the debug registers +of the inital breakpoint. + +Such code has to be marked with the 'noinstr' attribute. That places the +code into a special section which is taboo for instrumentation and debug +facilities. + +In a function which is marked 'noinstr' it's only allowed to call into +non-instrumentable code except when the invocation of instrumentable code +is annotated with a instrumentation_begin()/instrumentation_end() pair:: + + noinstr void entry(void) + { + handle_entry(); <-- must be 'noinstr' or '__always_inline' + ... + instrumentation_begin(); + handle_context(); <-- instrumentable code + instrumentation_end(); + ... + handle_exit(); <-- must be 'noinstr' or '__always_inline' + } + +This allows verification of the 'noinstr' restrictions via objtool on +supported architectures. + +Invoking non-instrumentable functions from instrumentable context has no +restrictions and is useful to protect e.g. state switching which would +cause malfunction if instrumented. + +All non-instrumentable entry/exit code sections before and after the RCU +state transitions must run with interrupts disabled. + +Syscalls +-------- + +Syscall entry exit code starts obviously in low level architecture specific +assembly code and calls out into C-code after establishing low level +architecture specific state and stack frames. This low level code must not +be instrumented. A typical syscall handling function invoked from low level +assembly code looks like this:: + + noinstr void do_syscall(struct pt_regs \*regs, int nr) + { + arch_syscall_enter(regs); + nr = syscall_enter_from_user_mode(regs, nr); + + instrumentation_begin(); + + if (!invoke_syscall(regs, nr) && nr != -1) + result_reg(regs) = __sys_ni_syscall(regs); + + instrumentation_end(); + + syscall_exit_to_user_mode(regs); + } + +syscall_enter_from_user_mode() first invokes enter_from_user_mode() which +establishes state in the following order: + + * Lockdep + * RCU / Context tracking + * Tracing + +and then invokes the various entry work functions like ptrace, seccomp, +audit, syscall tracing etc. After the function returns instrumentable code +can be invoked. After returning from the syscall handler the instrumentable +code section ends and syscall_exit_to_user_mode() is invoked. + +syscall_exit_to_user_mode() handles all work which needs to be done before +returning to user space like tracing, audit, signals, task work etc. After +that it invokes exit_to_user_mode() which again handles the state +transition in the reverse order: + + * Tracing + * RCU / Context tracking + * Lockdep + +syscall_enter_from_user_mode() and syscall_exit_to_user_mode() are also +available as fine grained subfunctions in cases where the architecture code +has to do extra work between the various steps. In such cases it has to +ensure that enter_from_user_mode() is called first on entry and +exit_to_user_mode() is called last on exit. + + +KVM +--- + +Entering or exiting guest mode is very similar to syscalls. From the host +kernel point of view the CPU goes off into user space when entering the +guest and returns to the kernel on exit. + +kvm_guest_enter_irqoff() is a KVM specific variant of exit_to_user_mode() +and kvm_guest_exit_irqoff() is the KVM variant of enter_from_user_mode(). +The state operations have the same ordering. + +Task work handling is done separately for guest at the boundary of the +vcpu_run() loop via xfer_to_guest_mode_handle_work() which is a subset of +the work handled on return to user space. + +Interrupts and regular exceptions +--------------------------------- + +Interrupts entry and exit handling is slightly more complex than syscalls +and KVM transitions. + +If an interrupt is raised while the CPU executes in user space, the entry +and exit handling is exactly the same as for syscalls. + +If the interrupt is raised while the CPU executes in kernel space the entry +and exit handling is slightly different. RCU state is only updated when the +interrupt was raised in context of the idle task because that's the only +kernel context where RCU can be not watching on NOHZ enabled kernels. +Lockdep and tracing have to be updated unconditionally. + +irqentry_enter() and irqentry_exit() provide the implementation for this. + +The architecture specific part looks similar to syscall handling:: + + noinstr void do_interrupt(struct pt_regs \*regs, int nr) + { + arch_interrupt_enter(regs); + state = irqentry_enter(regs); + + instrumentation_begin(); + + irq_enter_rcu(); + invoke_irq_handler(regs, nr); + irq_exit_rcu(); + + instrumentation_end(); + + irqentry_exit(regs, state); + } + +Note, that the invocation of the actual interrupt handler is within a +irq_enter_rcu() and irq_exit_rcu() pair. + +irq_enter_rcu() updates the preemption count which makes in_hardirq() +return true, handles NOHZ tick state and interrupt time accounting. This +means that up to the point where irq_enter_rcu() is invoked in_hardirq() +returns false. + +irq_exit_rcu() handles interrupt time accounting, undoes the preemption +count update and eventually handles soft interrupts and NOHZ tick state. + +The preemption count could be established in irqentry_enter() already, but +there is no real value to do so. This allows the preemption count to be +traced and just puts a restriction on the early entry code up to +irq_enter_rcu(). + +This also keeps the handling vs. irq_exit_rcu() symmetric and +irq_exit_rcu() must undo the preempt count elevation before handling soft +interrupts and irqentry_exit() also requires that because it might +schedule. + + +NMI and NMI-like exceptions +--------------------------- + +NMIs and NMI like exceptions, e.g. Machine checks, double faults, debug +interrupts etc. can hit any context and have to be extra careful vs. the +state. + +Debug exceptions can handle user space breakpoints or watchpoints in the +same way as an interrupt which was raised while executing in user space, +but kernel mode debug exceptions have to be treated like NMIs as they can +even happen in NMI context, e.g. due to code patching. + +Also Machine check exceptions can handle user mode exceptions like regular +interrupts, but for kernel mode exceptions they have to be treated like +NMIs. + +NMIs and the other NMI-like exceptions handle state transitions in the most +straight forward way and do not differentiate between user and kernel mode +origin. + +The state update on entry is handled in irqentry_nmi_enter() which updates +state in the following order: + + * Preemption counter + * Lockdep + * RCU + * Tracing + +The exit counterpart irqenttry_nmi_exit() does the reverse operation in the +reverse order. + +Note, that the update of the preemption counter has to be the first +operation on enter and the last operation on exit. The reason is that both +lockdep and RCU rely on in_nmi() returning true in this case. The +preemption count modification in the NMI entry/exit case can obviously not +be traced. + +Architecture specific code looks like this:: + + noinstr void do_nmi(struct pt_regs \*regs) + { + arch_nmi_enter(regs); + state = irqentry_nmi_enter(regs); + + instrumentation_begin(); + + invoke_nmi_handler(regs); + + instrumentation_end(); + irqentry_nmi_exit(regs); + } + +and for e.g. a debug exception it can look like this:: + + noinstr void do_debug(struct pt_regs \*regs) + { + arch_nmi_enter(regs); + + debug_regs = save_debug_regs(); + + if (user_mode(regs)) { + state = irqentry_enter(regs); + + instrumentation_begin(); + + user_mode_debug_handler(regs, debug_regs); + + instrumentation_end(); + + irqentry_exit(regs, state); + } else { + state = irqentry_nmi_enter(regs); + + instrumentation_begin(); + + kernel_mode_debug_handler(regs, debug_regs); + + instrumentation_end(); + + irqentry_nmi_exit(regs, state); + } + } + +There is no combined irqentry_nmi_if_kernel() function available as the +above cannot be handled in an exception agnostic way. --- a/Documentation/core-api/index.rst +++ b/Documentation/core-api/index.rst @@ -44,6 +44,14 @@ Library functionality that is used throu timekeeping errseq +Low level entry and exit +======================== + +.. toctree:: + :maxdepth: 1 + + entry + Concurrency primitives ====================== --- a/kernel/entry/common.c +++ b/kernel/entry/common.c @@ -1,5 +1,4 @@ // SPDX-License-Identifier: GPL-2.0 - #include #include #include