Re: [Xen-devel] [RFC] Support of non-indirect grant backend on 64KB guest

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 18/08/15 07:29, Julien Grall wrote:
> Hi,
> 
> Firstly, this patch is not ready at all and mostly here for
> collecting comment about the way to do it. It's not clean so no need
> to complain about the coding style.
> 
> The qdisk backend in QEMU is not supporting indirect grant, this is
> means that a request can only support 11 * 4KB = 44KB.
> 
> When using 64KB page, a Linux block request (struct *request) may
> contain up to 64KB of data. This is because the block segment size
> must at least be the size of a Linux page.

You should ensure you configure the request queue with the limits that
are currently supported.  In particular:

        /* Each segment in a request is up to an aligned page in
           size. */
        blk_queue_segment_boundary(rq, PAGE_SIZE - 1);
        blk_queue_max_segment_size(rq, PAGE_SIZE);

Is obviously wrong with PAGE_SIZE > 44 KiB.

Get the block later to split requests and don't do it in blkfront.

David

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