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

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Message-ID: <e7971d03-6ad1-40d5-9b79-f01242db5293@intel.com>
Date:   Mon, 18 Dec 2017 14:28:14 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ram Pai <linuxram@...ibm.com>
Cc:     mpe@...erman.id.au, mingo@...hat.com, akpm@...ux-foundation.org,
        corbet@....net, arnd@...db.de, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org, x86@...nel.org, linux-arch@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, benh@...nel.crashing.org,
        paulus@...ba.org, khandual@...ux.vnet.ibm.com,
        aneesh.kumar@...ux.vnet.ibm.com, bsingharora@...il.com,
        hbabu@...ibm.com, mhocko@...nel.org, bauerman@...ux.vnet.ibm.com,
        ebiederm@...ssion.com
Subject: Re: [PATCH v9 29/51] mm/mprotect, powerpc/mm/pkeys, x86/mm/pkeys: Add
 sysfs interface

On 12/18/2017 02:18 PM, Ram Pai wrote:
> b) minimum number of keys available to the application.
> 	if libraries consumes a few, they could provide a library
> 	interface to the application informing the number available to
> 	the application.  The library interface can leverage (b) to
> 	provide the information.

OK, let's see a real user of this including a few libraries.  Then we'll
put it in the kernel.

> c) types of disable-rights supported by keys.
> 	Helps the application to determine the types of disable-features
> 	available. This is helpful, otherwise the app has to 
> 	make pkey_alloc() call with the corresponding parameter set
> 	and see if it suceeds or fails. Painful from an application
> 	point of view, in my opinion.

Again, let's see a real-world use of this.  How does it look?  How does
an app "fall back" if it can't set a restriction the way it wants to?

Are we *sure* that such an interface makes sense?  For instance, will it
be possible for some keys to be execute-disable while other are only
write-disable?

> 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?"