linux-kernel - Re: [PATCH v9 29/51] mm/mprotect, powerpc/mm/pkeys, x86/mm/pkeys: Add sysfs interface

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Date:   Tue, 19 Dec 2017 08:22:21 -0800
From:   Ram Pai <linuxram@...ibm.com>
To:     Gabriel Paubert <paubert@...m.es>
Cc:     linux-arch@...r.kernel.org, corbet@....net, arnd@...db.de,
        linux-doc@...r.kernel.org, aneesh.kumar@...ux.vnet.ibm.com,
        x86@...nel.org, linux-kernel@...r.kernel.org, mhocko@...nel.org,
        linux-mm@...ck.org, Dave Hansen <dave.hansen@...el.com>,
        mingo@...hat.com, paulus@...ba.org, ebiederm@...ssion.com,
        linux-kselftest@...r.kernel.org, bauerman@...ux.vnet.ibm.com,
        akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        khandual@...ux.vnet.ibm.com
Subject: Re: [PATCH v9 29/51] mm/mprotect, powerpc/mm/pkeys, x86/mm/pkeys:
 Add sysfs interface

On Tue, Dec 19, 2017 at 09:31:22AM +0100, Gabriel Paubert wrote:
> On Mon, Dec 18, 2017 at 03:15:51PM -0800, Ram Pai wrote:
> > On Mon, Dec 18, 2017 at 02:28:14PM -0800, Dave Hansen wrote:
> > > On 12/18/2017 02:18 PM, Ram Pai wrote:
> > > 
....snip...
> > > > I think on x86 you look for some hardware registers to determine
> > > > which hardware features are enabled by the kernel.
> > > 
> > > No, we use CPUID.  It's a part of the ISA that's designed for
> > > enumerating CPU and (sometimes) OS support for CPU features.
> > > 
> > > > We do not have generic support for something like that on ppc.  The
> > > > kernel looks at the device tree to determine what hardware features
> > > > are available. But does not have mechanism to tell the hardware to
> > > > track which of its features are currently enabled/used by the
> > > > kernel; atleast not for the memory-key feature.
> > > 
> > > Bummer.  You're missing out.
> > > 
> > > But, you could still do this with a syscall.  "Hey, kernel, do you
> > > support this feature?"
> > 
> > or do powerpc specific sysfs interface.
> > or a debugfs interface.
> 
> getauxval(3) ?
> 
> With AT_HWCAP or HWCAP2 as parameter already gives information about
> features supported by the hardware and the kernel.
> 
> Taking one bit to expose the availability of protection keys to
> applications does not look impossible.
> 
> Do I miss something obvious?

No. I am told this is possible aswell.

RP