Xen devel - Jan 2007 - using PageForeign for pte allocation vs. shortcutting __pte_free_tlb()

In the course of trying to get CONFIG_HIGHPTE to work I stumbled across this
inconsistency between i386 and x86-64 Linux: i386 uses SetPageForeign() in
(one the non-highpte) case of pte_alloc_one(), and leaves intact
native''s
definition of __pte_free_tlb(), whereas x86-64 doesn''t use the former
but
shortcuts the latter to just invoke pte_free(). Obviously it would be nice for
these two to be consistent, the question just is which of the two behaviors
is more desirable: Obviously, performance wise using the PageForeign
mechanism would be preferable, but delta-to-native-wise I would think this
is the less desirable solution.

Thanks for any thoughts or suggestions,
Jan

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Jan Beulich wrote:
> In the course of trying to get CONFIG_HIGHPTE to work I stumbled across
this
> inconsistency between i386 and x86-64 Linux: i386 uses SetPageForeign() in
> (one the non-highpte) case of pte_alloc_one(), and leaves intact
native''s
> definition of __pte_free_tlb(), whereas x86-64 doesn''t use the
former but
> shortcuts the latter to just invoke pte_free(). Obviously it would be nice
for
> these two to be consistent,
x86-64 has no HIGHMEM zone, thus CONFIG_HIGHPTE has no effect (I''m
surprised it exists in the first place ...).

cheers,
  Gerd

-- 
Gerd Hoffmann <kraxel@suse.de>

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>>> Gerd Hoffmann <kraxel@suse.de> 09.01.07 11:51 >>>
>Jan Beulich wrote:
>> In the course of trying to get CONFIG_HIGHPTE to work I stumbled across
this
>> inconsistency between i386 and x86-64 Linux: i386 uses SetPageForeign()
in
>> (one the non-highpte) case of pte_alloc_one(), and leaves intact
native''s
>> definition of __pte_free_tlb(), whereas x86-64 doesn''t use the
former but
>> shortcuts the latter to just invoke pte_free(). Obviously it would be
nice for
>> these two to be consistent,
>
>x86-64 has no HIGHMEM zone, thus CONFIG_HIGHPTE has no effect (I''m
>surprised it exists in the first place ...).
Of course (and no, such a config option doesn''t exist for x86-64), but
what
relation does this have to the question asked?

Jan

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Jan Beulich wrote:
>>>> Gerd Hoffmann <kraxel@suse.de> 09.01.07 11:51
>>>
>> Jan Beulich wrote:
>>> In the course of trying to get CONFIG_HIGHPTE to work I stumbled
across this
>>> inconsistency between i386 and x86-64 Linux: i386 uses
SetPageForeign() in
>>> (one the non-highpte) case of pte_alloc_one(), and leaves intact
native''s
>>> definition of __pte_free_tlb(), whereas x86-64 doesn''t use
the former but
>>> shortcuts the latter to just invoke pte_free(). Obviously it would
be nice for
>>> these two to be consistent,
>> x86-64 has no HIGHMEM zone, thus CONFIG_HIGHPTE has no effect
(I''m
>> surprised it exists in the first place ...).
> 
> Of course (and no, such a config option doesn''t exist for x86-64),
but what
> relation does this have to the question asked?
guess (not having looked at the code at all) i386 does that because it
needs disturgish high and non-high ptes, whereas x86-64 simply has no
need for that, so I don''t see the point in trying to make them
consistent ...

cheers,
  Gerd

-- 
Gerd Hoffmann <kraxel@suse.de>

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>>> Gerd Hoffmann <kraxel@suse.de> 09.01.07 12:11 >>>
>Jan Beulich wrote:
>>>>> Gerd Hoffmann <kraxel@suse.de> 09.01.07 11:51
>>>
>>> Jan Beulich wrote:
>>>> In the course of trying to get CONFIG_HIGHPTE to work I
stumbled across this
>>>> inconsistency between i386 and x86-64 Linux: i386 uses
SetPageForeign() in
>>>> (one the non-highpte) case of pte_alloc_one(), and leaves
intact native''s
>>>> definition of __pte_free_tlb(), whereas x86-64 doesn''t
use the former but
>>>> shortcuts the latter to just invoke pte_free(). Obviously it
would be nice for
>>>> these two to be consistent,
>>> x86-64 has no HIGHMEM zone, thus CONFIG_HIGHPTE has no effect
(I''m
>>> surprised it exists in the first place ...).
>> 
>> Of course (and no, such a config option doesn''t exist for
x86-64), but what
>> relation does this have to the question asked?
>
>guess (not having looked at the code at all) i386 does that because it
>needs disturgish high and non-high ptes, whereas x86-64 simply has no
>need for that, so I don''t see the point in trying to make them
>consistent ...
I''m pretty sure that''s not the reason, partly because
CONFIG_HIGHPTE (as
indicated here and earlier) isn''t being an option for Xen kernels so
far at all,
but mostly because as I understand it both mechanisms are ways to
remove the write protection before handing a former page table page back
to the page allocator (and this requirement is clearly the same on both i386
and x86-64).

Jan

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