Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq_segments about VMERGE

[Boaz Harrosh <bharrosh@panasas.com>]

FUJITA Tomonori wrote:
> On Thu, 17 Jul 2008 07:50:24 -0400 (EDT)
> Mikulas Patocka <mpatocka@redhat.com> wrote:
> 
>>>>> Please give me an example how the boundary restriction of a device can
>>>>> break the VMERGE accounting and an IOMMU if you aren't still sure.
>>>> You have dma_get_seg_boundary and dma_get_max_seg_size. On sparc64, adding 
>>>> one of these broken VMERGE accounting (the VMERGE didn't happen past 64-kb 
>>>> boundary and bio layer thought that VMERGE would be possible).
>>> If the device has 64KB boundary restriction, the device also has
>>> max_seg_size restriction of 64KB or under. So the vmerge acounting
>>> works (though we need to fix it to handle max_seg_size, as discussed).
>>>
>>>> And if you fix this case, someone will break it again, sooner or later, by 
>>>> adding new restriction.
>>> All restrictions that IOMMUs need to know are dma_get_seg_boundary and
>>> dma_get_max_seg_size.
>>>
>>> What is your new restriction?
>> We don't know what happens in the future.
> 
> It's very unlikely to add new restrictions.
> 
> 
>> And that is the problem that we 
>> don't know --- but we have two pieces of code (blk-merge and iommu) that 
>> try to calculate the same number (number of hw segments) and if they get 
>> different result, it will crash. If the calculations were done at one 
>> place, there would be no problem with that.
> 
> I don't think that your argument, 'the problem that we don't know', is
> true.
> 
> With the vmerge accounting, we calculate at two places. So if we add
> a new restriction, we need to handle it at two places. It's a logical
> result.
> 
> Of course, it's easier to calculate at one place rather than two
> places. But 'we don't know what restriction we will need' isn't a
> problem.
> 
> 
> BTW, as I've already said, I'm not against removing the vmerge
> accounting from the block layer.

I have a question. Does the block layer know of the IOMMU in use
for the device? can it call into the IOMMU to calculate the
restriction?

Thanks Boaz