Re: [Xen-devel] CONFIG_XEN_COMPAT_030002 broken?

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 14/11/06 15:11, "Gerd Hoffmann" <kraxel@xxxxxxx> wrote:

>> not nice for use in (early) feature detection. Maybe it'd be better to try
>> and write
>> a page table entry without PAGE_USER, and check whether that bit got turned
>> on implicitly...
> 
> Patch attached, seems to work ok, survived quick test with ttylinux on
> both 3.0.3 and 3.0.2 without crashing and detected both versions correctly.
> 
> cheers,
>   Gerd

Kernel_pages_need_user_flag seems unnecessary. The one consumer of that flag
could simply do 'flags |= kernel_page_user' unconditionally.

Also, is it necessary to default to 3.0.2 behaviour? Could we have
kernel_page_user==0 initially and then change the value only if 3.0.2 is
detected? This would provide a sanity check that check_page_user_flag() is
being executed early enough. We could even set kernel_page_user to a garbage
value initially, like ~0.

So far we have maintained the COMPAT code to be easily entirely strippable.
So the change to pmd_bad() should either use kernel_page_user rather than
_PAGE_USER, or its definition should be conditional on the COMPAT flag.

 -- Keir


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