Re: [PATCH v8 4/4] riscv: Set unaligned access speed at compile time

[Charlie Jenkins]

On Fri, Mar 08, 2024 at 05:35:03AM -0800, Emil Renner Berthing wrote:
> Conor Dooley wrote:
> > On Fri, Mar 08, 2024 at 01:52:24AM -0800, Emil Renner Berthing wrote:
> > > Charlie Jenkins wrote:
> >
> > > >  config RISCV_MISALIGNED
> > > > -	bool "Support misaligned load/store traps for kernel and userspace"
> > > > +	bool
> > > >  	select SYSCTL_ARCH_UNALIGN_ALLOW
> > > > -	default y
> > > >  	help
> > > > -	  Say Y here if you want the kernel to embed support for misaligned
> > > > -	  load/store for both kernel and userspace. When disable, misaligned
> > > > -	  accesses will generate SIGBUS in userspace and panic in kernel.
> > > > +	  Embed support for misaligned load/store for both kernel and userspace.
> > > > +	  When disabled, misaligned accesses will generate SIGBUS in userspace
> > > > +	  and panic in the kernel.
> > >
> > > Hmm.. this is *may* generate SIGBUS in userspace and panic the kernel. The CPU
> > > could support unaligned access natively or there might be a handler in M-mode,
> > > right?
> >
> > Correct. The last sentence could become "When disabled, and there is no
> > support in hardware or firmware, unsigned accesses will...". That said,
> > this option is no longer user visible, so we could really simplify the
> > hell out of this option to just mention that it controls building the
> > in-kernel emulator.
> >
> > > > +choice
> > > > +	prompt "Unaligned Accesses Support"
> > > > +	default RISCV_PROBE_UNALIGNED_ACCESS
> > > > +	help
> > > > +	  This determines the level of support for unaligned accesses. This
> > > > +	  information is used by the kernel to perform optimizations. It is also
> > > > +	  exposed to user space via the hwprobe syscall. The hardware will be
> > > > +	  probed at boot by default.
> > > > +
> > > > +config RISCV_PROBE_UNALIGNED_ACCESS
> > > > +	bool "Probe for hardware unaligned access support"
> > > > +	select RISCV_MISALIGNED
> > > > +	help
> > > > +	  During boot, the kernel will run a series of tests to determine the
> > > > +	  speed of unaligned accesses. This probing will dynamically determine
> > > > +	  the speed of unaligned accesses on the underlying system. If unaligned
> > > > +	  memory accesses trap into the kernel as they are not supported by the
> > > > +	  system, the kernel will emulate the unaligned accesses to preserve the
> > > > +	  UABI.
> > > > +
> > > > +config RISCV_EMULATED_UNALIGNED_ACCESS
> > > > +	bool "Emulate unaligned access where system support is missing"
> > > > +	select RISCV_MISALIGNED
> > > > +	help
> > > > +	  If unaligned memory accesses trap into the kernel as they are not
> > > > +	  supported by the system, the kernel will emulate the unaligned
> > > > +	  accesses to preserve the UABI. When the underlying system does support
> > > > +	  unaligned accesses, the unaligned accesses are assumed to be slow.
> > >
> > > It's still not quite clear to me when you'd want to choose this over probing
> > > above. Assuming the probe measures correctly this can only result in a kernel
> > > that behaves the same or slower than with the option above, right?
> >
> > Aye, mostly the same - some people don't want the boot-time overhead
> > of actually running the profiling code, so this option is for them.
> > Maybe that's not such a big deal anymore with the improvements to do it
> > in parallel, but given how bad performance on some of the systems is
> > when firmware does the emulation, it is definitely still noticeable.
> > I know we definitely have customers that care about their boot time very
> > strongly, so I can imagine they'd be turning this off.
> 
> Ah, that makes sense. So maybe a help text more along the lines of "Disable
> probing and optimizations for CPUs with fast unaligned memory access" would be
> a better description of this choice?

It does cause probing/optimizations to not be enabled, but it does not
"disable" them. For maximal optimizations for fast unaligned accesses,
the user must select RISCV_EFFICIENT_UNALIGNED_ACCESS before and after
this change. For probing, the user must select
RISCV_PROBE_UNALIGNED_ACCESS.

- Charlie

> 
> > > > +
> > > > +config RISCV_SLOW_UNALIGNED_ACCESS
> > > > +	bool "Assume the system supports slow unaligned memory accesses"
> > > > +	depends on NONPORTABLE
> > > > +	help
> > > > +	  Assume that the system supports slow unaligned memory accesses. The
> > > > +	  kernel and userspace programs may not be able to run at all on systems
> > > > +	  that do not support unaligned memory accesses.
> > >
> > > Again you're just explicitly saying no to the optimizations the kernel might do
> > > if it detects fast unaligned access, only here you'll also crash if they're not
> > > handled by the CPU or M-mode. Why would you want that?
> > >
> > > I'm probably missing something, but the only reason I can think of is if you
> > > want build a really small kernel and save the few bytes for the handler and
> > > probing code.
> >
> > Aye, just to allow you to disable the in-kernel emulator. That's
> > currently a choice that is presented to people, so this option preserves
> > that. IMO this is by far the least useful option and is locked behind
> > NONPORTABLE anyway. Maybe we could delete it, and if someone really wants
> > it, it would not be all that much of a hassle to add back in the future?
> 
> Yeah, if noone really needs this less config options is better, but I don't
> feel strongly about this option either way.
> 
> /Emil