Re: [PATCH v3 15/20] block/xen-blkfront: Make it running on 64KB page granularity

From: Julien Grall
Date: Fri Aug 28 2015 - 11:35:29 EST

On 20/08/15 09:10, Roger Pau Monnà wrote:
> Hello,


> I have some comments regarding the commit message, IMHO it would be good
> that a native English speaker reviews it too.
> El 07/08/15 a les 18.46, Julien Grall ha escrit:
>> The PV block protocol is using 4KB page granularity. The goal of this
>> patch is to allow a Linux using 64KB page granularity using block
>> device on a non-modified Xen.
>> The block API is using segment which should at least be the size of a
>> Linux page. Therefore, the driver will have to break the page in chunk
>> of 4K before giving the page to the backend.
>> Breaking a 64KB segment in 4KB chunk will result to have some chunk with
>> no data.
> I would rewrite the above line as:
> When breaking a 64KB segment into 4KB chunks it is possible that some
> chunks are empty.

Sounds good, I will replace with it.

>> As the PV protocol always require to have data in the chunk, we
>> have to count the number of Xen page which will be in use and avoid to
> ^pages
>> sent empty chunk.
> ^and avoid sending empty chunks.
>> Note that, a pre-defined number of grant is reserved before preparing
> ^grants are
>> the request. This pre-defined number is based on the number and the
>> maximum size of the segments. If each segment contain a very small
> ^contains
>> amount of data, the driver may reserve too much grant (16 grant is
> ^many grants ^grants are
>> reserved per segment with 64KB page granularity).
>> Futhermore, in the case of persistent grant we allocate one Linux page
> ^grants
>> per grant although only the 4KB of the page will be effectively use.
> ^first ^in
>> This could be improved by share the page with multiple grants.
> ^sharing
>> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> Acked-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>

Thank you, I will fix all the typos in the next version.

> Just one question.


>> + gnttab_foreach_grant_in_range(sg_page(sg),
>> + sg->offset,
>> + sg->length,
>> + blkif_setup_rw_req_grant,
>> + &setup);
> If I'm understanding this right, on x86 gnttab_foreach_grant_in_range is
> only going to perform one iteration, since XEN_PAGE_SIZE == PAGE_SIZE.

Correct, it will only perform when iteration for x86 but also for arm32
and arm64 (when 4KB page is in use).


Julien Grall
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