On Thu, 30 May 2024 at 21:11, Sasha Levin <sashal@xxxxxxxxxx> wrote: > > This is a note to let you know that I've just added the patch titled > > arm64: fpsimd: Drop unneeded 'busy' flag > > to the 6.6-stable tree Why? > which can be found at: > http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary > > The filename of the patch is: > arm64-fpsimd-drop-unneeded-busy-flag.patch > and it can be found in the queue-6.6 subdirectory. > > If you, or anyone else, feels it should not be added to the stable tree, > please let <stable@xxxxxxxxxxxxxxx> know about it. > > > > commit 37f2773a1ef05374538d5e4ed26cbacebe363241 > Author: Ard Biesheuvel <ardb@xxxxxxxxxx> > Date: Fri Dec 8 12:32:20 2023 +0100 > > arm64: fpsimd: Drop unneeded 'busy' flag > > [ Upstream commit 9b19700e623f96222c69ecb2adecb1a3e3664cc0 ] > > Kernel mode NEON will preserve the user mode FPSIMD state by saving it > into the task struct before clobbering the registers. In order to avoid > the need for preserving kernel mode state too, we disallow nested use of > kernel mode NEON, i..e, use in softirq context while the interrupted > task context was using kernel mode NEON too. > > Originally, this policy was implemented using a per-CPU flag which was > exposed via may_use_simd(), requiring the users of the kernel mode NEON > to deal with the possibility that it might return false, and having NEON > and non-NEON code paths. This policy was changed by commit > 13150149aa6ded1 ("arm64: fpsimd: run kernel mode NEON with softirqs > disabled"), and now, softirq processing is disabled entirely instead, > and so may_use_simd() can never fail when called from task or softirq > context. > > This means we can drop the fpsimd_context_busy flag entirely, and > instead, ensure that we disable softirq processing in places where we > formerly relied on the flag for preventing races in the FPSIMD preserve > routines. > > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> > Reviewed-by: Mark Brown <broonie@xxxxxxxxxx> > Tested-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx> > Link: https://lore.kernel.org/r/20231208113218.3001940-7-ardb@xxxxxxxxxx > [will: Folded in fix from CAMj1kXFhzbJRyWHELCivQW1yJaF=p07LLtbuyXYX3G1WtsdyQg@xxxxxxxxxxxxxx] > Signed-off-by: Will Deacon <will@xxxxxxxxxx> > Stable-dep-of: b8995a184170 ("Revert "arm64: fpsimd: Implement lazy restore for kernel mode FPSIMD"") > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> > > diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h > index 6a75d7ecdcaa2..8e86c9e70e483 100644 > --- a/arch/arm64/include/asm/simd.h > +++ b/arch/arm64/include/asm/simd.h > @@ -12,8 +12,6 @@ > #include <linux/preempt.h> > #include <linux/types.h> > > -DECLARE_PER_CPU(bool, fpsimd_context_busy); > - > #ifdef CONFIG_KERNEL_MODE_NEON > > /* > @@ -28,17 +26,10 @@ static __must_check inline bool may_use_simd(void) > /* > * We must make sure that the SVE has been initialized properly > * before using the SIMD in kernel. > - * fpsimd_context_busy is only set while preemption is disabled, > - * and is clear whenever preemption is enabled. Since > - * this_cpu_read() is atomic w.r.t. preemption, fpsimd_context_busy > - * cannot change under our feet -- if it's set we cannot be > - * migrated, and if it's clear we cannot be migrated to a CPU > - * where it is set. > */ > return !WARN_ON(!system_capabilities_finalized()) && > system_supports_fpsimd() && > - !in_hardirq() && !irqs_disabled() && !in_nmi() && > - !this_cpu_read(fpsimd_context_busy); > + !in_hardirq() && !irqs_disabled() && !in_nmi(); > } > > #else /* ! CONFIG_KERNEL_MODE_NEON */ > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c > index 5cdfcc9e3e54b..b805bdab284c4 100644 > --- a/arch/arm64/kernel/fpsimd.c > +++ b/arch/arm64/kernel/fpsimd.c > @@ -85,13 +85,13 @@ > * softirq kicks in. Upon vcpu_put(), KVM will save the vcpu FP state and > * flag the register state as invalid. > * > - * In order to allow softirq handlers to use FPSIMD, kernel_neon_begin() may > - * save the task's FPSIMD context back to task_struct from softirq context. > - * To prevent this from racing with the manipulation of the task's FPSIMD state > - * from task context and thereby corrupting the state, it is necessary to > - * protect any manipulation of a task's fpsimd_state or TIF_FOREIGN_FPSTATE > - * flag with {, __}get_cpu_fpsimd_context(). This will still allow softirqs to > - * run but prevent them to use FPSIMD. > + * In order to allow softirq handlers to use FPSIMD, kernel_neon_begin() may be > + * called from softirq context, which will save the task's FPSIMD context back > + * to task_struct. To prevent this from racing with the manipulation of the > + * task's FPSIMD state from task context and thereby corrupting the state, it > + * is necessary to protect any manipulation of a task's fpsimd_state or > + * TIF_FOREIGN_FPSTATE flag with get_cpu_fpsimd_context(), which will suspend > + * softirq servicing entirely until put_cpu_fpsimd_context() is called. > * > * For a certain task, the sequence may look something like this: > * - the task gets scheduled in; if both the task's fpsimd_cpu field > @@ -209,27 +209,14 @@ static inline void sme_free(struct task_struct *t) { } > > #endif > > -DEFINE_PER_CPU(bool, fpsimd_context_busy); > -EXPORT_PER_CPU_SYMBOL(fpsimd_context_busy); > - > static void fpsimd_bind_task_to_cpu(void); > > -static void __get_cpu_fpsimd_context(void) > -{ > - bool busy = __this_cpu_xchg(fpsimd_context_busy, true); > - > - WARN_ON(busy); > -} > - > /* > * Claim ownership of the CPU FPSIMD context for use by the calling context. > * > * The caller may freely manipulate the FPSIMD context metadata until > * put_cpu_fpsimd_context() is called. > * > - * The double-underscore version must only be called if you know the task > - * can't be preempted. > - * > * On RT kernels local_bh_disable() is not sufficient because it only > * serializes soft interrupt related sections via a local lock, but stays > * preemptible. Disabling preemption is the right choice here as bottom > @@ -242,14 +229,6 @@ static void get_cpu_fpsimd_context(void) > local_bh_disable(); > else > preempt_disable(); > - __get_cpu_fpsimd_context(); > -} > - > -static void __put_cpu_fpsimd_context(void) > -{ > - bool busy = __this_cpu_xchg(fpsimd_context_busy, false); > - > - WARN_ON(!busy); /* No matching get_cpu_fpsimd_context()? */ > } > > /* > @@ -261,18 +240,12 @@ static void __put_cpu_fpsimd_context(void) > */ > static void put_cpu_fpsimd_context(void) > { > - __put_cpu_fpsimd_context(); > if (!IS_ENABLED(CONFIG_PREEMPT_RT)) > local_bh_enable(); > else > preempt_enable(); > } > > -static bool have_cpu_fpsimd_context(void) > -{ > - return !preemptible() && __this_cpu_read(fpsimd_context_busy); > -} > - > unsigned int task_get_vl(const struct task_struct *task, enum vec_type type) > { > return task->thread.vl[type]; > @@ -383,7 +356,7 @@ static void task_fpsimd_load(void) > bool restore_ffr; > > WARN_ON(!system_supports_fpsimd()); > - WARN_ON(!have_cpu_fpsimd_context()); > + WARN_ON(preemptible()); > > if (system_supports_sve() || system_supports_sme()) { > switch (current->thread.fp_type) { > @@ -467,7 +440,7 @@ static void fpsimd_save(void) > unsigned int vl; > > WARN_ON(!system_supports_fpsimd()); > - WARN_ON(!have_cpu_fpsimd_context()); > + WARN_ON(preemptible()); > > if (test_thread_flag(TIF_FOREIGN_FPSTATE)) > return; > @@ -1583,7 +1556,7 @@ void fpsimd_thread_switch(struct task_struct *next) > if (!system_supports_fpsimd()) > return; > > - __get_cpu_fpsimd_context(); > + WARN_ON_ONCE(!irqs_disabled()); > > /* Save unsaved fpsimd state, if any: */ > fpsimd_save(); > @@ -1599,8 +1572,6 @@ void fpsimd_thread_switch(struct task_struct *next) > > update_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE, > wrong_task || wrong_cpu); > - > - __put_cpu_fpsimd_context(); > } > > static void fpsimd_flush_thread_vl(enum vec_type type) > @@ -1892,13 +1863,15 @@ static void fpsimd_flush_cpu_state(void) > */ > void fpsimd_save_and_flush_cpu_state(void) > { > + unsigned long flags; > + > if (!system_supports_fpsimd()) > return; > WARN_ON(preemptible()); > - __get_cpu_fpsimd_context(); > + local_irq_save(flags); > fpsimd_save(); > fpsimd_flush_cpu_state(); > - __put_cpu_fpsimd_context(); > + local_irq_restore(flags); > } > > #ifdef CONFIG_KERNEL_MODE_NEON
