Re: [PATCHv2] ARM64: Add AT_ARM64_MIDR to the aux vector

[Siarhei Siamashka]

On Wed, 2 Sep 2015 00:28:28 +0000
"Pinski, Andrew" <Andrew.Pinski@xxxxxxxxxxxxxxxxxx> wrote:

> > On Sep 2, 2015, at 3:13 AM, Siarhei Siamashka <siarhei.siamashka@xxxxxxxxx> wrote:
> > 
> > On Wed, 2 Sep 2015 01:58:56 +0800
> > pinskia@xxxxxxxxx wrote:
> > 
> >>> On Sep 2, 2015, at 1:30 AM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> >>> 
> >>> [...]
> >>> 
> >>>>>>> On Sat, Aug 29, 2015 at 07:46:22PM +0100, Andrew Pinski wrote:
> >>>>>>> It is useful to pass down MIDR register down to userland if all of
> >>>>>>> the online cores are all the same type.  This adds AT_ARM64_MIDR
> >>>>>>> aux vector type and passes down the midr system register.
> >>>>>>> 
> >>>>>>> This is alternative to MIDR_EL1 part of
> >>>>>>> http://lists.infradead.org/pipermail/linux-arm-kernel/2015-July/358995.html.
> >>>>>>> It allows for faster access to midr_el1 than going through a trap and
> >>>>>>> does not exist if the set of cores are not the same.
> >>>>>> 
> >>>>>> I'm not sure I follow the rationale. If speed is important the
> >>>>>> application can cache the value the first time it reads it with a trap.
> >>>>> 
> >>>>> It is also about compatibility also. Exposing the register is not backwards compatible but using the aux vector is.
> >>>> 
> >>>> That would also break big.little too. So either break it with hot plug or break it in userland, your choice.
> >>> 
> >>> The value wouldn't be representative of the system as a whole; that is
> >>> true. However, we never guaranteed that it was, while the aux vector
> >>> code implied that we did.
> >> 
> >> Yes but I guess you talk about caching the value in userspace but doing
> >> it via the aux vector is the same as your suggestion. Just one
> >> difference is you don't get the aux vector entry if there is a CPU
> >> that is online which is different. No difference from your suggestion
> >> of caching it. Without considering hot pug for a second (that is a
> >> huge different issue all together), if userland wants to know if all
> >> up CPUs have the same midr, they would either read /sys entries (lots
> >> of syscalls) or bind to each CPU and do the trap. That means at least
> >> three or two syscalls/traps for each CPU. My way is none and gets a
> >> value of midr if they are all the Same for free. 
> > 
> > Andrew, how do you propose to get the value of MIDR? Open the
> > "/proc/self/auxv", read it, do a linear search in the buffer to find
> > the required entry and then read the value? Or use the glibc specific
> > getauxval() function (https://lwn.net/Articles/519085) ?
> 
> This is inside glibc I am talking about so getauxval. 

OK. Point taken.

> > Regarding the caching implementation, one can open and parse the
> > "/proc/cpuinfo" file (with older kernels) or read the new sysfs
> > entries to get the MIDR value for each core. Then create a lookup
> > table. As an additional bonus, this lookup table can contain not
> > just the MIDR values, but any arbitrary data in any format (for
> > example, a function pointer to the memcpy function or anything else).
> 
> You don't want to do that early on in ld.so each time a program
> starts up. Too much overhead. 

Right.

> > After the lookup table is available, one can use the getcpu() syscall
> > for getting the CPU number and do the table lookup. And for getting
> > reasonable performance, implement the vdso variant of the getcpu()
> > syscall.
> > 
> > All of this internal ugliness would be best abstracted inside
> > of the GCC __builtin_cpu_init(), __builtin_cpu_is() and
> > __builtin_cpu_supports() builtins:
> >    http://gcc.gnu.org/gcc-4.8/changes.html
> 
> Yes but this is about glibc support and not other userland
> support. Having glibc depend on that is even worse. 

Well, basically you are saying that you only care about your
individual use case (glibc only, 64-bit only, no support for
big.LITTLE) and just don't give rats about anything else. This
is not necessarily wrong, but we are not getting the problem
solved on a wider scale with such approach.

Regarding the reliance on the getauxval() function. Appears that
at least it seems to be also supported in recent versions of Android
and in musl too. So it already has a wide support in the Linux
different flavours and just needs a configure check in regular
applications/libraries. This is not perfect, but not bad either
and will only get better in the future.

Regarding the performance. Avoiding to open and read any files
surely helps to make the setup cost lower. In this sense, the
    http://lists.infradead.org/pipermail/linux-arm-kernel/2015-July/359127.html
patch is not very good because we would have to deal with multiple
files, increasing the overhead even more.

Regarding the big.LITTLE hardware. We can't just ignore it and
pretend that it does not exist. This would be rather short-sighted.
Just like not thinking about allowing userspace access to the MIDR
register was a short-sighted decision in the past for ARMv8 and
earlier architectures.

So let's review everything. You are suggesting that we should try
to read AT_ARM64_MIDR via getauxval (btw, why ARM64 in the name?) and
if it is found, then all the CPU cores are identical and we have the
MIDR value. If it is not found, then we should check AT_HWCAP for the
HWCAP_CPUID bit and read the MIDR register directly (which will be
trapped, but emulated in the kernel). 

The HWCAP_CPUID approach and emulation looks interesting, because it
might probably motivate ARM to actually implement direct userspace
access to the MIDR register in the future revisions of hardware :-)

Now I wonder about the performance difference between the kernel
emulated MIDR read and the performance of doing a table lookup, using
sched_getcpu() result as an index. And I have done some preliminary
tests. On my Cortex-A7 processor, running "sched_getcpu()" in a loop
takes ~200 cycles per iteration. Running "getauxval(AT_PLATFORM)" in
the same loop takes ~100 cycles per iteration. The emulated SWP and
unaligned LDM instructions take around ~500 and ~400 cycles. But the
emulation of the MIDR register read should be much more lightweight
than SWP/LDM, so it might be more competitive with sched_getcpu()
and I will benchmark this later.

I wonder if we still can try to make "sched_getcpu()" use vDSO
instead of the syscall to make it faster? Now that there exists a
vDSO implementation for gettimeofday(), everything should be easy if
we can find an unused userspace readable coprocessor register.
In the past, Catalin Marinas mentioned that "We have a user read-only
thread register unused on arm64":
    http://lists.infradead.org/pipermail/linux-arm-kernel/2013-December/220664.html
And if I understand it correctly, also one of the "scratch registers"
should exist in 32-bit ARM, which "isn't present in ARMv8/AArch64":
    http://lists.infradead.org/pipermail/linux-arm-kernel/2013-December/221056.html
What kind of registers are these exactly?

In principle, the aux vector can be extended to also contain a pointer
to an array of MIDR values for all the CPU cores if reducing the setup
overhead is critical.

The GCC __builtin_cpu_is() and __builtin_cpu_supports() builtins are
unrelated to the kernel and not really needed for glibc, but it's
important to ensure that they can be implemented efficiently on ARM.
It would be great if we could delegate all this ugly and hairy
runtime CPU features detection task to GCC. Instead of implementing
it in every application and library, which makes use of assembly
optimizations.

-- 
Best regards,
Siarhei Siamashka
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