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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

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

> --- 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.

Jan


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