Re: [Xen-devel] [PATCH 0001/001] xen: multi page ring support for block devices

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.03.12 at 07:32, Justin Gibbs <justing@xxxxxxxxxxxxxxxx> wrote:
> There's another problem here that I brought up during the Xen
> Hack-a-thon.  The ring macros require that the ring element count
> be a power of two.  This doesn't mean that the ring will be a power
> of 2 pages in size.  To illustrate this point, I modified the FreeBSD
> blkback driver to provide negotiated ring stats via sysctl.
> 
> Here's a connection to a Windows VM running the Citrix PV drivers:
> 
>     dev.xbbd.2.max_requests: 128
>     dev.xbbd.2.max_request_segments: 11
>     dev.xbbd.2.max_request_size: 45056
>     dev.xbbd.2.ring_elem_size: 108  <= 32bit ABI
>     dev.xbbd.2.ring_pages: 4
>     dev.xbbd.2.ring_elements: 128
>     dev.xbbd.2.ring_waste: 2496
> 
> Over half a page is wasted when ring-page-order is 2.  I'm sure you
> can see where this is going.  :-)

Having looked a little closer on how the wasted space is progressing,
I find myself in the odd position that I can't explain the original (and
still active) definition of BLKIF_MAX_SEGMENTS_PER_REQUEST (11):
With ring-order zero, there's 0x240/0x1c0 bytes (32/64-bit
respectively) are unused. With 32 requests fitting in the ring, and with
each segment occupying 6 bytes (padded to 8), in the 64-bit variant
there's enough space for a 12th segment (32-bit would even have
space for a 13th). Am I missing anything here?

Plus all this assumes a page size of 4k, yet ia64 had always been using
pages of 16k iirc.

> Here are the limits published by our backend to the XenStore:
>
>     max-ring-pages = "113"
>     max-ring-page-order = "7"
>     max-requests = "256"
>     max-request-segments = "129"
>     max-request-size = "524288"

Oh, so this protocol doesn't require ring-pages (and max-ring-pages)
to be a power of two? In which case I think it is a mistake to also
advertise max-ring-page-order, as at least the (Linux) frontend code
I know of interprets this as being able to set up a ring of (using the
numbers above) 128 pages (unless, of course, your backend can deal
with this regardless of the max-ring-pages value it announces).

Jan


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