On Wed, Oct 21, 2020 at 06:39:00PM +0100, Will Deacon wrote: > On Wed, Oct 21, 2020 at 05:52:47PM +0100, Catalin Marinas wrote: > > On Wed, Oct 21, 2020 at 05:21:21PM +0100, Qais Yousef wrote: > > > On 10/21/20 16:39, Will Deacon wrote: > > > > On Wed, Oct 21, 2020 at 11:46:09AM +0100, Qais Yousef wrote: > > > > > When the CONFIG_ASYMMETRIC_AARCH32 option is enabled (EXPERT), the type > > > > > of the ARM64_HAS_32BIT_EL0 capability becomes WEAK_LOCAL_CPU_FEATURE. > > > > > The kernel will now return true for system_supports_32bit_el0() and > > > > > checks 32-bit tasks are affined to AArch32 capable CPUs only in > > > > > do_notify_resume(). If the affinity contains a non-capable AArch32 CPU, > > > > > the tasks will get SIGKILLed. If the last CPU supporting 32-bit is > > > > > offlined, the kernel will SIGKILL any scheduled 32-bit tasks (the > > > > > alternative is to prevent offlining through a new .cpu_disable feature > > > > > entry). > > > > > > > > > > In addition to the relaxation of the ARM64_HAS_32BIT_EL0 capability, > > > > > this patch factors out the 32-bit cpuinfo and features setting into > > > > > separate functions: __cpuinfo_store_cpu_32bit(), > > > > > init_cpu_32bit_features(). The cpuinfo of the booting CPU > > > > > (boot_cpu_data) is now updated on the first 32-bit capable CPU even if > > > > > it is a secondary one. The ID_AA64PFR0_EL0_64BIT_ONLY feature is relaxed > > > > > to FTR_NONSTRICT and FTR_HIGHER_SAFE when the asymmetric AArch32 support > > > > > is enabled. The compat_elf_hwcaps are only verified for the > > > > > AArch32-capable CPUs to still allow hotplugging AArch64-only CPUs. > > > > > > > > > > Make sure that KVM never sees the asymmetric 32bit system. Guest can > > > > > still ignore ID registers and force run 32bit at EL0. > > > > > > > > > > Co-developed-by: Qais Yousef <qais.yousef@xxxxxxx> > > > > > Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx> > > > > > Signed-off-by: Qais Yousef <qais.yousef@xxxxxxx> > > > > > > > > [...] > > > > > > > > > diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h > > > > > index 5e784e16ee89..312974ab2c85 100644 > > > > > --- a/arch/arm64/include/asm/thread_info.h > > > > > +++ b/arch/arm64/include/asm/thread_info.h > > > > > @@ -67,6 +67,7 @@ void arch_release_task_struct(struct task_struct *tsk); > > > > > #define TIF_FOREIGN_FPSTATE 3 /* CPU's FP state is not current's */ > > > > > #define TIF_UPROBE 4 /* uprobe breakpoint or singlestep */ > > > > > #define TIF_FSCHECK 5 /* Check FS is USER_DS on return */ > > > > > +#define TIF_CHECK_32BIT_AFFINITY 6 /* Check thread affinity for asymmetric AArch32 */ > > > > > > > > I've looked through the patch and I still can't figure out why this extra > > > > flag is needed. We know if a CPU supports 32-bit EL0, and we know whether > > > > or not a task is 32-bit. So why the extra flag? Is it just a hangover from > > > > the old series? > > > > > > It did evolve a bit organically. > > > > > > AFAICS it helps as an optimization to avoid the checks unnecessarily. If it's > > > not expensive to do the checks in the loop in do_notify_resume() we can omit > > > it. We will still protect it with system_supports_asym_32bit_el0() so the check > > > is done on these systems only. > > > > Ah, I think I remember now. We didn't want ret_to_user (entry.S) to > > always go the work_pending path if there was no context switch for a > > 32-bit task. With the AArch32 check in do_notify_resume(), it would mean > > we add _TIF_32BIT to the _TIF_WORK_MASK. > > > > However, we could add an asm alternative if AArch32 asym is detected to > > always route TIF_32BIT tasks to work_pending. > > Or could we just use TIF_NOTIFY_RESUME, like we for for rseq()? Maybe but it needs some analysis to make sure we don't lose this flag. It is cleared in some places. -- Catalin
