Re: [XenPPC] Re: [Xen-ia64-devel] [PATCH 1/2] remove xencomm page size limit(xen side)

[Hollis Blanchard <hollisb@xxxxxxxxxx>]

On Wed, 2007-08-01 at 15:36 +0900, Isaku Yamahata wrote:
> remove xencomm page size limit.
> Currently xencomm has page size limit so that a domain with many memory
> (e.g. 100GB+) can't be created.
> This patch allows that the address array of struct xencomm_desc to cross
> page boundary so that the size of struct xencomm_desc can exceed page size.
> Note that struct xencomm_desc itself can't page boundary. 

I'm confused about this.

We noticed a much smaller limit doing save/restore. The p2m mapping
maxed out at 512MB:
      * a xencomm_desc must fit into 1 page (1<<12)
      * it contains approx 1<<9 64-bit entries
      * each entry points to a page of memory containing 1<<9 entries of
        the p2m array
      * total: 1<<18 p2m entries
      * each p2m entry represents a guest page (1<<12)
      * 1<<30 total guest memory can be represented with a single
        xencomm_desc

But we can issue sequential p2m hcalls with different offsets, so we do.
So what exactly is the new problem?


Secondly, your patch doesn't make much sense to me, because the Xen side
is explicitly rejecting xencomm_descs that cross a page boundary. So
even if Linux sends down a multipage descriptor, Xen will return EINVAL.
Am I missing something?

(As a side question, is it really so difficult for you guys to just use
the common code? I tried very hard to make it easy for you to just
implement a couple hooks; I can't imagine it would take more than a day
to do it.)

-- 
Hollis Blanchard
IBM Linux Technology Center


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