Re: [PATCH] xen/memguard: Drop memguard_init() entirely

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 22/02/16 10:02, Jan Beulich wrote:
>>>> On 19.02.16 at 17:18, <andrew.cooper3@citrix.com> wrote:
>> On 19/02/16 14:44, Jan Beulich wrote:
>>>>>> On 18.02.16 at 19:03, <andrew.cooper3@citrix.com> wrote:
>>>> It is not obvious what this code is doing.  Most of it dates from 2007/2008,
>>>> and there have been substantial changes in Xen's memory handling since then.
>>> Deleting code which isn't understood what it is or was once used
>>> for is sub-optimal.
>>>
>>>> It was previously optional, and isn't needed for any of the memguard
>>>> infrastructure to function.  The use of MAP_SMALL_PAGES causes needless
>>>> shattering of superpages.
>>> Perhaps that's what is its purpose? Let's ask Keir, whom you didn't
>>> even Cc.
>> I don't see this patch being different in nature to your "x86: drop
>> failsafe callback invocation from assembly".
> That other patch explains why the code is (and never was)
> necessary, whereas you just guess.
>
>> As I explain in the second paragraph, these calls are strictly optional,
>> as they are omitted for release builds.  They also have no impact on the
>> rest of the memguard infrastructure to function, as
>> __memguard_change_range() also uses map_pages_to_xen().
>>
>> So despite not being sure why it is like it is, I am stating with that
>> it is not needed with Xen in its current form.
> I actually think the reason is to avoid the memory allocation which
> might result the first time a 2M page gets split up, as that memory
> allocation might fail (which nowadays gets a proper -ENOMEM
> communicated out of map_pages_to_xen(), but that hasn't been
> the case in the early days).

And isn't the case anywhere in the memguard infrastructure.

At the end of this series, there is no shattering of any superpages in
the normal .text/data/bss region, but there is guarding of the pcpu
stack which is liable to shatter specific superpages mapping the
directmap region.

~Andrew