Re: [Xen-devel] [PATCH 14/17] xen/arm64: head: Remove ID map as soon as it is not used

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: <xen-devel@lists.xenproject.org>
Subject: Re: [Xen-devel] [PATCH 14/17] xen/arm64: head: Remove ID map as soon as it is not used
Date: Wed, 26 Jun 2019 21:44:09 +0100	[thread overview]
Message-ID: <86de69b7-b18c-1285-de94-89f18137ca0f@citrix.com> (raw)
In-Reply-To: <12426eb9-1545-b1f7-a85a-55c9e04b8cc4@arm.com>

On 26/06/2019 21:39, Julien Grall wrote:
> On 6/26/19 9:25 PM, Stefano Stabellini wrote:
>> On Mon, 10 Jun 2019, Julien Grall wrote:
>>> The ID map may clash with other parts of the Xen virtual memory layout.
>>> At the moment, Xen is handling the clash by only creating a mapping to
>>> the runtime virtual address before enabling the MMU.
>>>
>>> The rest of the mappings (such as the fixmap) will be mapped after the
>>> MMU is enabled. However, the code doing the mapping is not safe as it
>>> replace mapping without using the Break-Before-Make sequence.
>>>
>>> As the ID map can be anywhere in the memory, it is easier to remove all
>>> the entries added as soon as the ID map is not used rather than adding
>>> the Break-Before-Make sequence everywhere.
>>
>> I think it is a good idea, but I would ask you to mention 1:1 map
>> instead of "ID map" in comments and commit messages because that is the
>> wording we used in all comments so far (see the one in
>> create_page_tables and mm.c). It is easier to grep and refer to if we
>> use the same nomenclature. Note that I don't care about which
>> nomenclature we decide to use, I am only asking for consistency.
>> Otherwise, it would also work if you say it both way at least once:
>>
>>   The ID map (1:1 map) may clash [...]
>
> I would rather drop the wording 1:1 as this is confusing. It is also
> not trivial to find anything on google typing "1:1".

"one-to-one mapping", or "identity map" are both common terminology. 
1:1 is a common representation for the former, whereas ID is not a
abbreviation of "Identity".

If you don't want to use 1:1, then you need to say "The identity map" to
retain clarity.

~Andrew

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