linux-kernel - Re: [RFC v6 21/62] powerpc: introduce execute-only pkey

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Date:   Tue, 01 Aug 2017 16:46:22 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
        Ram Pai <linuxram@...ibm.com>
Cc:     linux-arch@...r.kernel.org, corbet@....net, arnd@...db.de,
        linux-doc@...r.kernel.org, x86@...nel.org, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, mhocko@...nel.org,
        linux-mm@...ck.org, mingo@...hat.com, paulus@...ba.org,
        aneesh.kumar@...ux.vnet.ibm.com, linux-kselftest@...r.kernel.org,
        akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        khandual@...ux.vnet.ibm.com
Subject: Re: [RFC v6 21/62] powerpc: introduce execute-only pkey

Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com> writes:
> Ram Pai <linuxram@...ibm.com> writes:
...
>> +
>> +	/* We got one, store it and use it from here on out */
>> +	if (need_to_set_mm_pkey)
>> +		mm->context.execute_only_pkey = execute_only_pkey;
>> +	return execute_only_pkey;
>> +}
>
> If you follow the code flow in __execute_only_pkey, the AMR and UAMOR
> are read 3 times in total, and AMR is written twice. IAMR is read and
> written twice. Since they are SPRs and access to them is slow (or isn't
> it?),

SPRs read/writes are slow, but they're not *that* slow in comparison to
a system call (which I think is where this code is being called?).

So we should try to avoid too many SPR read/writes, but at the same time
we can accept more than the minimum if it makes the code much easier to
follow.

cheers