On Tue, Dec 19, 2017 at 09:50:24PM +1100, Michael Ellerman wrote:

Dave Hansen [off-list ref] writes:

quoted

On 11/06/2017 12:57 AM, Ram Pai wrote:

quoted

Expose useful information for programs using memory protection keys.
Provide implementation for powerpc and x86.

On a powerpc system with pkeys support, here is what is shown:

$ head /sys/kernel/mm/protection_keys/*
==> /sys/kernel/mm/protection_keys/disable_access_supported <==
true

This is cute, but I don't think it should be part of the ABI.  Put it in
debugfs if you want it for cute tests.  The stuff that this tells you
can and should come from pkey_alloc() for the ABI.

Yeah I agree this is not sysfs material.

In particular the total/usable numbers are completely useless vs other
threads allocating pkeys out from under you.

The usable number is the minimum number of keys available for use by the
application, not the number of keys **currently** available.  Its a
static number.

I am dropping this patch. We can revisit this when a clear request for
such a feature emerges.

quoted

quoted

       Any application wanting to use protection keys needs to be able to
       function without them.  They might be unavailable because the
       hardware that the application runs on does not support them, the
       kernel code does not contain support, the kernel support has been
       disabled, or because the keys have all been allocated, perhaps by a
       library the application is using.  It is recommended that
       applications wanting to use protection keys should simply call
       pkey_alloc(2) and test whether the call succeeds, instead of
       attempting to detect support for the feature in any other way.

Do you really not have standard way on ppc to say whether hardware
features are supported by the kernel?  For instance, how do you know if
a given set of registers are known to and are being context-switched by
the kernel?

Yes we do, we emit feature bits in the AT_HWCAP entry of the aux vector,
same as some other architectures.

Ah. I was not aware of this.
Thanks,
RP