Re: [PATCH 0/2] Add support for enhanced atomic operations

[Vladimir Sokolovsky <vlad-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org>]

Sean Hefty wrote:
>> This patchset adds support for the following enhanced atomic operations:
>> - Masked atomic compare and swap
>> - Masked atomic fetch and add
> 
> Can you explain these in more detail?  How exactly is a mask used?  Are there
> any restrictions on the format of the mask?  How is the mask carried over the
> wire?  etc.
> 
> --

Masked Compare and Swap (MskCmpSwap)
The MskCmpSwap atomic operation is an extension to the CmpSwap operation defined in the IB
spec. MskCmpSwap allows the user to select a portion of the 64 bit target data for the “compare”
check as well as to restrict the swap to a (possibly different) portion. The pseudo code below
describes the operation:

-----
atomic_response = *va
if (((cmp XOR *va) AND cmp_mask) is ZERO) then
	*va = (*va AND NOT(swap_mask)) OR (swap AND swap_mask)

return atomic_response
-----

The additional operands are carried in the Extended Transport Header. Atomic response generation
and packet format for MskCmpSwap is as for standard IB Atomic operations.


Masked Fetch and Add (MFetchAdd)
The MFetchAdd Atomic operation extends the functionality of the standard IB FetchAdd by
allowing the user to split the target into multiple fields of selectable length. The atomic add is done
independently on each one of this fields. A bit set in the field_boundary parameter specifies the
field boundaries. The pseudo code below describes the operation:

------------
bit_adder(ci, b1, b2, *co)
{
         value = ci + b1 + b2;

         if (value & 2)
                 *co = 1
         else
                 *co = 0

         return value & 1
}

mask = 1
carry = 0
atomic_response = *va
#define MASK_IS_SET(mask, attr)      (((mask)&(attr))!=0)

for i = 0 to 63
{
         if ( i != 0 )
                 mask =  mask << 1;

         bit_add_res = bit_adder(carry, MASK_IS_SET(atomic_response, mask), MASK_IS_SET(add_value, mask), &new_carry)
         if (bit_add_res)
                 atomic_response |= mask

         carry = ((new_carry) && (!MASK_IS_SET(cmp_mask, mask)))
}

return atomic_response
------------

The additional operands are carried in the Extended Transport Header. Atomic response generation
and packet format for MFetchAdd is as for standard IB Atomic operations.

Regards,
Vladimir
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