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Re: [Xen-devel] blkfront/back and sector size of 2048 with CDROM's

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] blkfront/back and sector size of 2048 with CDROM's
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Tue, 05 Feb 2008 09:03:13 +0000
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Thread-topic: [Xen-devel] blkfront/back and sector size of 2048 with CDROM's
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On 5/2/08 08:52, "James Harper" <james.harper@xxxxxxxxxxxxxxxx> wrote:

> Okay... a forinstance...
> I give windows a sector size of 2048 based on what I have read from
> xenstore. Windows isssues a read request for sector 1 (byte offset
> 2048), and a length of 1 sector.
> My read buffer is aligned at 1024 bytes into the page, so in the blkif
> request I set nr_segments = 1, sector_number = 4 (1 * (2048 / 512)), and
> in the first (and only) segment, first_sect = 2 and last_sect = 6.
> Does that sound right?

Yes, except that last_sect = 5, because it's an inclusive range.

I'm pretty sure you can DMA to/from unaligned buffers (e.g., in this case
non-2kB aligned). Certainly blkback doesn't appear to check for it, so
presumably we found it works!

 -- Keir

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