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

[Roger Pau Monnà <roger.pau@xxxxxxxxxx>]

Hello,

El 18/08/15 a les 8.29, Julien Grall ha escrit:
> 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.
> 
> So when indirect is not supported by the backend, we are not able to fit all 
> the data in a single request. We therefore need to create a second request to 
> copy the rest of the data.
> 
> I've wrote a patch last week which make 64KB guest booting with qdisk. 
> Although, I'm not sure this is the right way to do it. I would appreciate if 
> one of the block maintainers give me insight about it.

Maybe I'm missing some key data, but I see two ways to solve this, the
first one is the one you describe above, and consists in allowing
blkfront to split a request into multiple ring slots. The other solution
would be to add indirect descriptors support to Qdisk, has this been
looked into?

AFAICT it looks more interesting, and x86 can also benefit from it.
Since I would like to prevent adding more cruft to blkfront, I rather
prefer 64KB guests to require indirect descriptors in order to run.

Roger.


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