Re: [Xen-devel] [PATCH v2 5/9] x86/pagewalk: Helpers for reserved bit handling

[Juergen Gross <jgross@xxxxxxxx>]

On 24/03/17 08:51, Jan Beulich wrote:
>>>> On 24.03.17 at 06:45, <jgross@xxxxxxxx> wrote:
>> On 23/03/17 18:35, Andrew Cooper wrote:
>>> Would you prefer ~((uint64_t)_PAGE_PSE_PAT | (_PAGE_PSE_PAT - 1)) or
>>> ~(_PAGE_PSE_PAT | (_PAGE_PSE_PAT - 1) | 0ULL)
>>
>> Wouldn't it be better to just define the _PAGE_PSE bits accordingly?
> 
> I don't think that's a good idea, since the flags accessors deal with
> unsigned int quantities (see {get,put}_pte_flags()), and there's no
> need to promote these to 64 bit operations. Otherwise you'd also
> have to e.g. ask for _PAGE_NX_BIT to be made 1ULL << 63 instead
> of its current 1ULL << 23.

Well, I came to my conclusion looking at the usage of _PAGE_PSE_PAT
in do_recalc() (source file arch/x86/mm/p2m-pt.c). While being fine
right now it will be a problem as soon as we support >16 TB hosts.
And finding these kind of problems might be hard.

Juergen


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