From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCH] introduce grant copy for user land
Date: Tue, 11 Nov 2014 15:06:44 +0000
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To: Thanos Makatos <thanos.makatos@citrix.com>, "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
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On 11/11/14 14:42, Thanos Makatos wrote:
>> On 11/11/14 12:27, Thanos Makatos wrote:
>>>> The arbitrary limitations in number of ops and page alignment should
>>>> be removed.  I think both can be removed relatively easily by
>>>> consuming page aligned chunks of segments and doign the hypercall
>>>> when a batch of ops is filled.
>>>
>>> Wouldn't this lead to multiple calls to gnttab_batch_copy(), potentially
>> hurting performance?
>>
>> The incremental benefits of batching diminishes as the batch size increase.
> 
> To overcome the page alignment limitation, struct gntdev_grant_copy_segment
> will have to contain a set of grant refs instead of just one grant ref,
> correct?

Oh, that would be awkward.  Keeping the alignment requirement would be best.

>> We also don't want multi-page allocations in this driver, so the batch size
>> should be set accordingly.
> 
> Why not?

Multi-page allocations are more likely to fail because of memory
fragmentation.

>> The interface should also support grant to grant copies.
> 
> To support grant to grant copies, struct gntdev_grant_copy_segment will have
> to contain an additional set of grant refs, correct?

The hypervisor ABI uses a union of handle and grant ref.  Perhaps
something similar for the kernel interface?

David