Re: [PATCH v2 3/3] tools: introduce parameter max_wp_ram_ranges.

["Yu, Zhang" <yu.c.zhang@linux.intel.com>]

On 1/27/2016 11:12 PM, Jan Beulich wrote:
>>>> On 27.01.16 at 15:56, <yu.c.zhang@linux.intel.com> wrote:
>> On 1/27/2016 10:32 PM, Jan Beulich wrote:
>>>>>> On 27.01.16 at 15:13, <yu.c.zhang@linux.intel.com> wrote:
>>>> About the truncation issue:
>>>>      I do not quite follow. Will this hurt if the value configured does
>>>> not exceed 4G? What about a type cast?
>>>
>>> A typecast would not alter behavior in any way. And of course
>>> a problem only arises if the value was above 4 billion. You either
>>> need to refuse such values while the attempt is made to set it.
>>> or you need to deal with the full range of possible values. Likely
>>> the former is the better (and I wonder whether the upper
>>> bound shouldn't be forced even lower than 4 billion).
>>
>> Oh, I see. A check with the upper bound sounds better. Using 4G as the
>> upper bound is a little conservative, but I do not have any better
>> criteria right now. :)
>
> But when making that decision keep security in mind: How much
> memory would it take to populate 4G rangeset nodes?
>
Well, for XenGT, one extreme case I can imagine would be half of all
the guest ram is used as the GPU page table, and page frames containing
these page tables are discontinuous (rangeset can combine continuous
ranges). For other virtual devices to leverage the write-protected gfn
rangeset, I believe the same idea applies. :)
Is this logic OK?

Thanks
Yu