Message-ID: <554BC1DC.3050303@sr71.net>
Date: Thu, 07 May 2015 12:49:48 -0700
From: Dave Hansen <dave@sr71.net>
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To: Christian Borntraeger <borntraeger@de.ibm.com>,
        Ingo Molnar <mingo@kernel.org>
CC: linux-kernel@vger.kernel.org, x86@kernel.org,
        linux-s390 <linux-s390@vger.kernel.org>
Subject: Re: [PATCH 00/12] [RFC] x86: Memory Protection Keys
References: <20150507174132.34AF8FAF@viggo.jf.intel.com> <20150507175707.GA22172@gmail.com> <554BAA68.6000508@sr71.net> <554BBB74.70706@de.ibm.com> <554BBD0F.6080209@sr71.net> <554BC0CE.6070900@de.ibm.com>
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On 05/07/2015 12:45 PM, Christian Borntraeger wrote:
>>> >> This all looks like s390 storage keys (with the key in pagetables instead
>>> >> of a dedicated place). There we also have 16 values for the key and 4 bits 
>>> >> in the PSW that describe the thread local key both are matched.
>>> >> There is an additional field F (fetch protection) that decides, if the
>>> >> key value is used for stores or for stores+fetches.
>> > 
>> > OK, so a thread can only be in one domain at a time?
> Via the PSW yes.
> Actually the docs talk about access key, which is usually the PSW. There are
> some instructions like MOVE WITH KEY that allow to specify the key for this
> specific instruction. For compiled code these insructions are not used in 
> Linux and I can not really see a way to implement that properly. Furthermore
> enabling these key ops has other implications which are unwanted.

OK, so we have to basic operations that need to be done for
protection/storage/$FOO keys:

1. Assign a key (or set of keys) to a memory area
2. Have a thread request the access (read and/or write) to a set of
   areas be acquired or revoked.

For (2) on x86, we basically allow any combination of keys and r/w
permissions.  On s390, we would need to ensure that acces to only one
key was allowed at a time.

BTW, do the s390 keys affect instructions and data, or data only?

The x86 ones affect data only.

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