On Wed, Oct 21, 2020 at 03:09:46PM +0100, Catalin Marinas wrote: > On Wed, Oct 21, 2020 at 03:33:29PM +0200, Morten Rasmussen wrote: > > On Wed, Oct 21, 2020 at 01:15:59PM +0100, Catalin Marinas wrote: > > > one, though not as easy as automatic task placement by the scheduler (my > > > first preference, followed by the id_* regs and the aarch32 mask, though > > > not a strong preference for any). > > > > Automatic task placement by the scheduler would mean giving up the > > requirement that the user-space affinity mask must always be honoured. > > Is that on the table? > > I think Peter rejected it but I still find it a nicer interface from a > dumb application perspective. It may interact badly with cpusets though > (at least on Android). > > > Killing aarch32 tasks with an empty intersection between the > > user-space mask and aarch32_mask is not really "automatic" and would > > require the aarch32 capability to be exposed anyway. > > I agree, especially if overriding the user mask is not desirable. But if > one doesn't play around with cpusets, 32-bit apps would run "fine" with > the scheduler transparently placing them on the correct CPU. > > Anyway, if the task placement is entirely off the table, the next thing > is asking applications to set their own mask and kill them if they do > the wrong thing. Here I see two possibilities for killing an app: > > 1. When it ends up scheduled on a non-AArch32-capable CPU That sounds fine to me. If we could do the exception return and take a SIGILL, that's what we'd do, but we can't so we have to catch it before. > 2. If the user cpumask (bar the offline CPUs) is not a subset of the > aarch32_mask > > Option 1 is simpler but 2 would be slightly more consistent. I disagree -- if we did this for something like fpsimd, then the consistent behaviour would be to SIGILL on the cores without the instructions. > There's also the question on whether the kernel should allow an ELF32 to > be loaded (and potentially killed subsequently) if the user mask is not > correct on execve(). I don't see the point in distinguishing between "you did execve() on a core without 32-bit" and "you did execve() on a core with 32-bit and then migrated to a core without 32-bit". Will
