Re: [Xen-devel] [PATCH RFC 01/12] xen-blkback: don't store dev_bus_addr

[Roger Pau Monné]

On 28/02/13 11:58, Jan Beulich wrote:
>>>> On 28.02.13 at 11:28, Roger Pau Monne <roger.pau@xxxxxxxxxx> wrote:
>> dev_bus_addr returned in the grant ref map operation is the mfn of the
>> passed page, there's no need to store it in the persistent grant
>> entry, since we can always get it provided that we have the page.
> 
> Interesting that you come up with this, as I have a similar patch
> pending (not posted yet), aiming at reducing the stack usage in
> dispatch_rw_block_io(): seg[].buf is really unnecessary with the
> dev_bus_addr storing removed, as the only reader of that field
> can equally well use req->u.rw.seg[i].first_sect.

Well, it can if we are not using indirect descriptors, because once we
start using indirect descriptors segments are inside a gref frame, so
it's quite comfortable to store first_sect inside a separate array, this
way we can map indirect segments, copy whatever data we need from them
and unmap them, without having them around for the whole lifetime of the
request.

> 
> And then the biolist[] array really can be folded into a union
> with the remaining seg[] one, as their usage scopes are easily
> separable.

Could we leave that for a further patch? I would like to avoid messing
any more with blkback, as I'm already touching a lot of bits with this
patch series.

> 
>> --- a/drivers/block/xen-blkback/blkback.c
>> +++ b/drivers/block/xen-blkback/blkback.c
>> @@ -621,9 +621,7 @@ static int xen_blkbk_map(struct blkif_request *req,
>>  				 * If this is a new persistent grant
>>  				 * save the handler
>>  				 */
>> -				persistent_gnts[i]->handle = map[j].handle;
>> -				persistent_gnts[i]->dev_bus_addr =
>> -					map[j++].dev_bus_addr;
>> +				persistent_gnts[i]->handle = map[j++].handle;
>>  			}
>>  			pending_handle(pending_req, i) =
>>  				persistent_gnts[i]->handle;
>> @@ -631,7 +629,8 @@ static int xen_blkbk_map(struct blkif_request *req,
>>  			if (ret)
>>  				continue;
>>  
>> -			seg[i].buf = persistent_gnts[i]->dev_bus_addr |
>> +			seg[i].buf = pfn_to_mfn(page_to_pfn(
>> +				persistent_gnts[i]->page)) << PAGE_SHIFT |
> 
> So why do you do this? The only reader masks the field with
> ~PAGE_MASK anyway.

Yes, I only need to store first_sect.
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