Re: [Xen-devel] Converting heap page_infos to contiguous virtual

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/13/2016 04:17 PM, Boris Ostrovsky wrote:
> On 07/13/2016 04:02 PM, Andrew Cooper wrote:
>> On 13/07/16 20:44, Boris Ostrovsky wrote:
>>> I would like to clear a bunch of Xen heap pages at once (i.e. not
>>> page-by-page).
>>>
>>> Greatly simplifying things, let's say I grab (in common/page_alloc.c)
>>>     pg = page_list_remove_head(&heap(node, zone, order)
>>>
>>> and then
>>>
>>>     mfn_t mfn =
>>> _mfn(page_to_mfn(pg));                                        
>>>     char *va = mfn_to_virt(mfn_x(mfn));
>>>     memset(va, 0, 4096 * (1 << order));
>>>
>>>
>>> Would it be valid to this?
>> In principle, yes.  The frame_table is in order.
>>
>> However, mfn_to_virt() will blow up for RAM above the 5TB boundary.  You
>> need to map_domain_page() to get a mapping.
> Right, but that would mean going page-by-page, which I want to avoid.
>
> Now, DIRECTMAP_SIZE is ~128TB (if my math is correct) --- doesn't it
> imply that it maps this big a range contiguously (modulo PDX hole)?
>
>>>  Do I need to account for the PDX hole?
>> Jan is probably the best person to ask about this, but I am failure sure
>> there are lurking dragons here.
>>
>> PDX compression is used to reduce the size of the frametable when there
>> are large unused ranges of mfns.  Without paying attention to the PDX
>> shift, you don't know where the discontinuities lie.
>>
>> However, because the PDX shift is an aligned power of two, there are
>> likely to be struct page_info*'s in the frame_table which don't point at
>> real RAM, and won't have a virtual mapping even in the directmap.
> So I would be OK with finding which mfn of my range points to beginning
> of the hole and break the mfn range into two sections --- one below the
> hole and one above. With hope that both ranges can be mapped
> contiguously --- something that I don't know whether is true.


In fact, I am OK with breaking the range into many chunks. My goal is to
be able to map something like a few megabytes (maybe few tens of
megabytes at most) contiguously.

-boris


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