On Wed, Oct 21, 2020 at 04:03:13PM +0100, Qais Yousef wrote: > On 10/21/20 15:41, Will Deacon wrote: > > On Wed, Oct 21, 2020 at 01:15:59PM +0100, Catalin Marinas wrote: > > > On Wed, Oct 21, 2020 at 12:09:58PM +0100, Marc Zyngier wrote: > > > > On 2020-10-21 11:46, Qais Yousef wrote: > > > > > Example output. I was surprised that the 2nd field (bits[7:4]) is > > > > > printed out > > > > > although it's set as FTR_HIDDEN. > > > > > > > > > > # cat /sys/devices/system/cpu/cpu*/regs/identification/id_aa64pfr0 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > > > > > > # echo 1 > /proc/sys/kernel/enable_asym_32bit > > > > > > > > > > # cat /sys/devices/system/cpu/cpu*/regs/identification/id_aa64pfr0 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > 0x0000000000000012 > > > > > 0x0000000000000012 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > > > > This looks like a terrible userspace interface. It exposes unrelated > > > > features, > > > > > > Not sure why the EL1 field ended up in here, that's not relevant to the > > > user. > > > > > > > and doesn't expose the single useful information that the kernel has: > > > > the cpumask describing the CPUs supporting AArch32 at EL0. Why not expose > > > > this synthetic piece of information which requires very little effort from > > > > userspace and doesn't spit out unrelated stuff? > > > > > > I thought the whole idea is to try and avoid the "very little effort" > > > part ;). > > > > > > > Not to mention the discrepancy with what userspace gets while reading > > > > the same register via the MRS emulation. > > > > > > > > Granted, the cpumask doesn't fit the cpu*/regs/identification hierarchy, > > > > but I don't think this fits either. > > > > > > We already expose MIDR and REVIDR via the current sysfs interface. We > > > can expand it to include _all_ the other ID_* regs currently available > > > to user via the MRS emulation and we won't have to debate what a new > > > interface would look like. The MRS emulation and the sysfs info should > > > probably match, though that means we need to expose the > > > ID_AA64PFR0_EL1.EL0 field which we currently don't. > > > > > > I do agree that an AArch32 cpumask is an easier option both from the > > > kernel implementation perspective and from the application usability > > > 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). > > > > If a cpumask is easier to implement and easier to use, then I think that's > > what we should do. It's also then dead easy to disable if necessary by > > just returning 0. The only alternative I would prefer is not having to > > expose this information altogether, but I'm not sure that figuring this > > out from MIDR/REVIDR alone is reliable. > > I did suggest this before, but I'll try gain. If we want to assume a custom > bootloader and custom user space, we can make them provide the mask. Who mentioned a custom bootloader? In the context of Android, we're talking about a user-space that already manages scheduling affinity. > For example, the new sysctl_enable_asym_32bit could be a cpumask instead of > a bool as it currently is. Or we can make it a cmdline parameter too. > In both cases some admin (bootloader or init process) has to ensure to fill it > correctly for the target platform. The bootloader should be able to read the > registers to figure out the mask. So more weight to make it a cmdline param. I think this is adding complexity for the sake of it. I'm much more in favour of keeping the implementation and ABI as simple as possible: expose the fact that the system is heterogenous, have an opt-in for userspace to say it can handle that and let it handle it. Will
