Re: [SUGGESTION]: drop virtual merge accounting in I/O requests

[Mikulas Patocka]

On Fri, 11 Jul 2008, FUJITA Tomonori wrote:

> On Thu, 10 Jul 2008 17:56:08 -0400 (EDT)
> Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
> > When I thought about it more, I realized that this accounting of virtual
> > segments in I/O layer can't work correctly at all. If an architecture
> > defines symbols BIOVEC_VIRT_MERGEABLE and BIOVEC_VIRT_OVERSIZE, it
> > declares that it's IOMMU must merge any two regions satisfying these
> > conditions. But in an IOMMU, it is impossible to guarantee, because:
>
> Yeah, IOMMUs can't guarantee that. The majority of architectures set
> BIO_VMERGE_BOUNDARY to 0 so they don't hit this, I think.

Yes, the architectures without IOMMU don't hit this problem.

> > * the bus address is allocated basiclly randomly, so we can hit
> > dev->dma_parms->segment_boundary_mask any time. This will prevent virtual
> > merging from happenning. I/O layer doesn't know the bus address at the
> > time it merges requests, so it can't predict when this happens.
> >
> > * the IOMMU isn't guaranteed to find a continuous space in it's bus
> > address space. If it skips over already mapped regions, it can't perform
> > virtual merging.
> >
> > * when creating the mapping, we can hit per-device limit
> > "dev->dma_parms->max_segment_size" --- but the I/O layer checks only
> > against global limit BIOVEC_VIRT_OVERSIZE. (this last issue is fixable,
> > the previous two are not).
>
> I think that the block layer can handle this properly via
> q->max_segment_size. We have the same value at two different
> places. Yeah, it's not good...
>
>
> BTW, inia100_template sets sg_tablesize to SG_ALL. If the controller
> has at most 32 SG entries per request, we need to fix that.

Later, it sets that to shost->sg_tablesize = TOTAL_SG_ENTRY; I don't know 
why in inia100_template there is SG_ALL.

Mikulas
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