Messages in this thread | | | Date | Thu, 19 Oct 2023 15:50:53 +0100 | From | Mark Rutland <>, Will Deacon <> | Subject | Re: [PATCH 2/4] arm64/process: Make loading of 32bit processes depend on aarch32_enabled() |
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On Thu, Oct 19, 2023 at 04:32:27PM +0200, Andrea della Porta wrote: > On 15:27 Thu 19 Oct , Mark Rutland wrote: > > On Thu, Oct 19, 2023 at 02:38:32PM +0200, Andrea della Porta wrote: > > > On 13:52 Wed 18 Oct , Mark Rutland wrote: > > > > On Wed, Oct 18, 2023 at 01:13:20PM +0200, Andrea della Porta wrote: > > > > > Major aspect of Aarch32 emulation is the ability to load 32bit > > > > > processes. > > > > > That's currently decided (among others) by compat_elf_check_arch(). > > > > > > > > > > Make the macro use aarch32_enabled() to decide if Aarch32 compat is > > > > > enabled before loading a 32bit process. > > > > > > > > > > Signed-off-by: Andrea della Porta <andrea.porta@suse.com> > > > > > > > > Why can't you make system_supports_32bit_el0() take the option into account > > > > instead? > > > > > > > > > > I may be wrong here, but it seems to me that system_supports_32bit_el0() > > > answers teh question "can this system supports compat execution?" rather than > > > "do I want this system to run any compat execution?". That's the point of > > > aarch32_enabled(), to state whether we want teh system to run A32 code or not, > > > regardless of the system supporting it (of course, if the system does not > > > support A32 in EL0, this is a no-no, but that's another story). > > > > That's what the implementation does today, but we're really using it as a "do > > we intend for 32-bit EL0 to work?" predicate, and generally the > > system_supports_${FEATURE}() helpers are affected by the combination of actual > > HW support, kernel config options, *and* kernel command line options. For > > example, system_supports_sve() is affected by both CONFIG_ARM64_SVE and the > > "arm64.nosve" command line option. > > > > Thanks, > > Mark. > > Many thanks for the explanation, then inserting aach32_enabled() in > system_supports_32bit_el0() is the way to go.
I think what we should do here is clean up the way we implement system_supports_32bit_el0() such that it can be a cpucap, and have the conditions that would affect aarch32_enabled() feed into that. That way, system_supports_32bit_el0() will compile down to a single branch/nop (or elided entirely when known to be false at compile-time), and with that I think can reasonably fold the existing UNHANDLED() logic into the entry-common.c exception handlers as a simplification.
The only obviously painful part is that enable_mismatched_32bit_el0() allows (mismatched) AArch32 support to be enabled after we finalize system cpucaps, as part of a late hotplug. I suspect that was implemented that way for expedience rather than because we wanted to enable mismatched AArch32 after finalizing cpucaps.
Will, do you remember why we used a cpuhp callback for enabling mismatched 32-bit support? I couldn't see anything explicit in the commit message for:
2122a833316f2f3f ("arm64: Allow mismatched 32-bit EL0 support")
... and I suspect it was just easier to write that way, rather than adding more code around setup_system_capabilities() ?
Mark.
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