Re: [CapROS-devel] Proposed change to string passing

[Norman H. <no...@ca...>]

On 2007 Nov 8, at 4:26 PM, Charles Landau wrote:

> At 2:40 PM -0800 11/8/07, Norman Hardy wrote:
>> It looks like a rather clean design.
>> Any qualms have already been raised in other responses but I think we
>> should compare of the roles of the "Indirect Data capability" and the
>> segment capability.
>> Perhaps the wheel of reincarnation is turning.
>> http://cap-lore.com/Hardware/Wheel.html
>>
>> If you pass data by passing segments by passing segment keys then all
>> the copying may be avoided.
>> Are the drawbacks to segment keys for this role different than IDCs?
>
> I thought about segment keys while inventing IDCs. There are a couple
> of important differences:
>
> - With IDCs your data does not have to begin and end on a page =20
> boundary.
>
> - After an IDC is Released, the sender is guaranteed that the
> receiver's access to the data has been revoked. Segment keys can be
> revoked too but it takes many more key invocations.
>
> These seemed important enough to warrant a new mechanism.

The hope, at least in the IP problem, was to avoid all copies.
I agree that this complicates 'user mode' code in order to avoid =20
complicating kernel code.
If the ultimate objection is that NICs or DMAs demand contiguous =20
memory, then wrap them in an object that does the copy as it must to =20
suit the hardware requirements.
(By the way, what about DMA and page boundary crossings?)


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Norm Hardy: <http://cap-lore.com>

What has always made the State a hell on earth has been precisely =20
mankind's attempt to turn it into a paradise.
Friedrich H=F6lderlin (1770-1843)