On Tue, Dec 08, 2020 at 01:28:20PM +0000, Will Deacon wrote: > The aim of this series is to allow 32-bit ARM applications to run on > arm64 SoCs where not all of the CPUs support the 32-bit instruction set. > Unfortunately, such SoCs are real and will continue to be productised > over the next few years at least. I can assure you that I'm not just > doing this for fun. > > Changes in v5 include: > > * Teach cpuset_cpus_allowed() about task_cpu_possible_mask() so that > we can avoid returning incompatible CPUs for a given task. This > means that sched_setaffinity() can be used with larger masks (like > the online mask) from userspace and also allows us to take into > account the cpuset hierarchy when forcefully overriding the affinity > for a task on execve(). > > * Honour task_cpu_possible_mask() when attaching a task to a cpuset, > so that the resulting affinity mask does not contain any incompatible > CPUs (since it would be rejected by set_cpus_allowed_ptr() otherwise). > > * Moved overriding of the affinity mask into the scheduler core rather > than munge affinity masks directly in the architecture backend. Hurmph... so if I can still read, this thing will auto truncate the affinity mask to something that only contains compatible CPUs, right? Assuming our system has 8 CPUs (0xFF), half of which are 32bit capable (0x0F), then, when our native task (with affinity 0x3c) does a fork()+execve() of a 32bit thingy the resulting task has 0x0c. If that in turn does fork()+execve() of a native task, it will retain the trucated affinity mask (0x0c), instead of returning to the wider mask (0x3c). IOW, any (accidental or otherwise) trip through a 32bit helper, will destroy user state (the affinity mask: 0x3c). Should we perhaps split task_struct::cpus_mask, one to keep an original copy of the user state, and one to be an effective cpumask for the task? That way, the moment a task constricts or widens it's task_cpu_possible_mask() we can re-compute the effective mask without loss of information.
