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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.08.15 at 09:09, <roger.pau@xxxxxxxxxx> wrote:
> 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.

I was about to reply (almost) the same...

Jan


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