Re: [Xen-devel] [PATCH v2 2/8] xen/x86: Improvements to build-time pagetable generation

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.02.16 at 15:58, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 24/02/16 14:15, Jan Beulich wrote:
>>>>> On 24.02.16 at 14:57, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 24/02/16 11:24, Jan Beulich wrote:
>>>>  >>> On 23.02.16 at 17:31, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>>  GLOBAL(l1_identmap)
>>>>> -        pfn = 0
>>>>> +        idx = 0
>>>>>          .rept L1_PAGETABLE_ENTRIES
>>>>>          /* VGA hole (0xa0000-0xc0000) should be mapped UC. */
>>>>> -        .if pfn >= 0xa0 && pfn < 0xc0
>>>>> -        .long (pfn << PAGE_SHIFT) | PAGE_HYPERVISOR_NOCACHE | 
>>>>> MAP_SMALL_PAGES
>>>>> +        .if idx >= 0xa0 && idx < 0xc0
>>>>> +        .quad (idx << PAGE_SHIFT) | PAGE_HYPERVISOR_NOCACHE
>>>>>          .else
>>>>> -        .long (pfn << PAGE_SHIFT) | PAGE_HYPERVISOR | MAP_SMALL_PAGES
>>>>> +        .quad (idx << PAGE_SHIFT) | PAGE_HYPERVISOR
>>>> Please don't eliminate the MAP_SMALL_PAGES here, they serve an
>>>> (at least documentation) purpose.
>>> How?  Its in a l1 so are necessarily small pages, and the other l1's
>>> don't use the constant.
>> MAP_SMALL_PAGES documents (and enforces) that the mappings
>> shouldn't be re-combined into 2M ones, even if - after adjustments
>> to the other attributes - they could be.
> 
> In which case, is actively wrong.  Were the cacheabilities to change
> (e.g. booting HVMLite and knowing that there was no legacy VGA hole),
> the mappings should be recombined into a 2M superpage.

No, I think there are reasons (to do with fixed range MTRRs and
errata) which require us to keep these as small pages even if they
could be recombined.

Jan


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