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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.02.16 at 16:22, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 24/02/16 15:18, Jan Beulich wrote:
>>>>> 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)
> 
> Any idea about which generation this might apply to?

Just read the SDM sub-section "Large Page Size Considerations"
inside the section on MTRRs.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel