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

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Date:	Thu, 28 Feb 2013 10:58:46 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Roger Pau Monne" <roger.pau@...rix.com>
Cc:	<xen-devel@...ts.xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH RFC 01/12] xen-blkback: don't store
 dev_bus_addr

>>> On 28.02.13 at 11:28, Roger Pau Monne <roger.pau@...rix.com> 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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