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

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

On 24/02/16 14:15, Jan Beulich wrote:
>>>> On 24.02.16 at 14:57, <andrew.cooper3@citrix.com> wrote:
>> On 24/02/16 11:24, Jan Beulich wrote:
>>>  >>> On 23.02.16 at 17:31, <andrew.cooper3@citrix.com> 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.

~Andrew