Re: [Qemu-devel] [PATCH v2 06/12] Translate offsets to destination address space

["Michael R. Hines" <mrhines@linux.vnet.ibm.com>]

On 06/11/2015 01:58 PM, Dr. David Alan Gilbert wrote:
> * Michael R. Hines (mrhines@linux.vnet.ibm.com) wrote:
>> On 06/11/2015 12:17 PM, Dr. David Alan Gilbert (git) wrote:
>>> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>>>
>>> The 'offset' field in RDMACompress and 'current_addr' field
>>> in RDMARegister are commented as being offsets within a particular
>>> RAMBlock, however they appear to actually be offsets within the
>>> ram_addr_t space.
>>>
>>> The code currently assumes that the offsets on the source/destination
>>> match, this change removes the need for the assumption for these
>>> structures by translating the addresses into the ram_addr_t space of
>>> the destination host.
>>>
>>> Note: An alternative would be to change the fields to actually
>>> take the data they're commented for; this would potentially be
>>> simpler but would break stream compatibility for those cases
>>> that currently work.
>>>
>>> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>>> ---
>>>   migration/rdma.c | 31 ++++++++++++++++++++++++-------
>>>   1 file changed, 24 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/migration/rdma.c b/migration/rdma.c
>>> index 9532461..cb66721 100644
>>> --- a/migration/rdma.c
>>> +++ b/migration/rdma.c
>>> @@ -411,7 +411,7 @@ static void network_to_control(RDMAControlHeader *control)
>>>    */
>>>   typedef struct QEMU_PACKED {
>>>       union QEMU_PACKED {
>>> -        uint64_t current_addr;  /* offset into the ramblock of the chunk */
>>> +        uint64_t current_addr;  /* offset into the ram_addr_t space */
>>>           uint64_t chunk;         /* chunk to lookup if unregistering */
>>>       } key;
>>>       uint32_t current_index; /* which ramblock the chunk belongs to */
>>> @@ -419,8 +419,19 @@ typedef struct QEMU_PACKED {
>>>       uint64_t chunks;            /* how many sequential chunks to register */
>>>   } RDMARegister;
>>>
>>> -static void register_to_network(RDMARegister *reg)
>>> +static void register_to_network(RDMAContext *rdma, RDMARegister *reg)
>>>   {
>>> +    RDMALocalBlock *local_block;
>>> +    local_block  = &rdma->local_ram_blocks.block[reg->current_index];
>>> +
>>> +    if (local_block->is_ram_block) {
>>> +        /*
>>> +         * current_addr as passed in is an address in the local ram_addr_t
>>> +         * space, we need to translate this for the destination
>>> +         */
>>> +        reg->key.current_addr -= local_block->offset;
>>> +        reg->key.current_addr += rdma->dest_blocks[reg->current_index].offset;
>>> +    }
>>>       reg->key.current_addr = htonll(reg->key.current_addr);
>>>       reg->current_index = htonl(reg->current_index);
>>>       reg->chunks = htonll(reg->chunks);
>>> @@ -436,13 +447,19 @@ static void network_to_register(RDMARegister *reg)
>>>   typedef struct QEMU_PACKED {
>>>       uint32_t value;     /* if zero, we will madvise() */
>>>       uint32_t block_idx; /* which ram block index */
>>> -    uint64_t offset;    /* where in the remote ramblock this chunk */
>>> +    uint64_t offset;    /* Address in remote ram_addr_t space */
>>>       uint64_t length;    /* length of the chunk */
>>>   } RDMACompress;
>>>
>>> -static void compress_to_network(RDMACompress *comp)
>>> +static void compress_to_network(RDMAContext *rdma, RDMACompress *comp)
>>>   {
>>>       comp->value = htonl(comp->value);
>>> +    /*
>>> +     * comp->offset as passed in is an address in the local ram_addr_t
>>> +     * space, we need to translate this for the destination
>>> +     */
>>> +    comp->offset -= rdma->local_ram_blocks.block[comp->block_idx].offset;
>>> +    comp->offset += rdma->dest_blocks[comp->block_idx].offset;
>>>       comp->block_idx = htonl(comp->block_idx);
>>>       comp->offset = htonll(comp->offset);
>>>       comp->length = htonll(comp->length);
>> So, why add the destination block's offset on the source side
>> just for it to be re-adjusted again when it gets to the destination side?
>>
>> Can you just stop at this:
>>
>> +        reg->key.current_addr -= local_block->offset;
>>
>> Without this:
>>
>> +        reg->key.current_addr +=
>> rdma->dest_blocks[reg->current_index].offset;
>>
>> ... on the source, followed by this on the destionation:
>>
>> +    comp->offset -= rdma->local_ram_blocks.block[comp->block_idx].offset;
>>
>> Without this:
>>
>> +    comp->offset += rdma->dest_blocks[comp->block_idx].offset;
>>
>> Did I follow correctly?
> Aren't both of those conversions happening on the source?
> Anyway, I think what you're saying is that we change the value sent over
> the network to be an offset within the block instead of an offset in
> the whole ram_addr_t space (i.e. that's what happens if you don't
> add back on the dest_blocks[].offset).

Yes, right. Can you skip adding/subtracting the local block offset on 
each side?

- Michael