On Tue, Sep 01, 2015 at 05:51:44PM +0100, pinskia at gmail.com wrote:

quoted

On Sep 2, 2015, at 12:33 AM, Mark Rutland [off-list ref] wrote:

Hi,

quoted

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. 

So long as we have HWCAP_CPUID describing the availability of register
access [2], then userspace can test for that before attempting to access
the MIDR.

Other than that, I don't see a backwards or forwards compatibility
issue.

quoted

quoted

+u32 get_arm64_midr(void)
+{
+    int i;
+    u32 midr = 0;
+
+    for_each_online_cpu(i) {
+        struct cpuinfo_arm64 *cpuinfo = &per_cpu(cpu_data, i);
+        u32 oldmidr = midr;
+
+        midr = cpuinfo->reg_midr;
+        /*
+         * If there are cpus which have a different
+         * midr just return 0.
+         */
+        if (oldmidr && oldmidr != midr)
+            return 0;
+    }
+
+    return midr;
+}

If I have a big.LITTLE system where all the big CPUs are currently
offline, this will leave the MIDR the little CPUs in the auxvec.
However, at any point after this has run, I could hotplug the big CPUs
on and the little CPUs off, leaving this reporting a MIDR that
represents none of the online CPUs.

Given big.LITTLE and the potential for physical/dynamic hotplug (where
we won't know all the MIDRs in advance), I don't think that we can
generally expose a common MIDR in this fashion, and I don't think that
we should give the impression that we can.

This is standard issue with hot plug and big.little. Really big.little
is a design flaw but I am not going into that here. 

Regardless of our personal feelings on big.LITTLE, it's something we
have to deal with.

Hopefully it's a non-issue anyway; a MIDR provided by this interface can
really only be used to derive optimisation criteria rather than
non-architected properties required for correctness.

quoted

I think that the only things we can do are expose the MIDR for CPU the
code is currently executing on (as Suzuki's patches do), and/or expose
all the MIDRs for currently online CPUs (as Steve's [1] patch does).
Anything else leaves us trying to provide semantics that we cannot
guarantee.

Except they are not backwards compatible which means nobody in their
right mind would use the register to get the midr that way.

I assume you missed the discussion of HWCAP_CPUID, which prevents the
compatibility issue I believe you're considering here.

I am sorry but having a newer version of glibc working on a year old
kernel is not going to fly. 

I'm not sure I follow this, unless you meant _not_ working.

Thanks,
Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-July/359127.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-August/363559.html