Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 26 May 2006 04:55:22 -0400 | From | Jeff Garzik <> | Subject | Re: [discuss] Re: [PATCH 2/4] x86-64: Calgary IOMMU - move valid_dma_direction into the callers |
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Andi Kleen wrote: > On Thursday 25 May 2006 11:58, Jeff Garzik wrote: >> Muli Ben-Yehuda wrote: >>> On Thu, May 25, 2006 at 12:35:07AM -0400, Jeff Garzik wrote: >>>> Jon Mason wrote: >>>>> >From Andi Kleen's comments on the original Calgary patch, move >>>>> valid_dma_direction into the calling functions. >>>>> >>>>> Signed-off-by: Muli Ben-Yehuda <muli@il.ibm.com> >>>>> Signed-off-by: Jon Mason <jdmason@us.ibm.com> >>>> Even though BUG_ON() includes unlikely(), this introduces additional >>>> tests in very hot paths. >>> Are they really very hot? I mean if you're calling the DMA API, you're >>> about to frob the hardware or have already frobbed it - does this >>> check really matter? >> When you are adding a check that will _never_ be hit in production, to >> the _hottest_ paths in the kernel, you can be assured it matters... > > pci_dma_* shouldn't be that hot. Or at least IO usually has so much > overhead that some more bugging shouldn't matter.
I respectfully disagree with that logic. If its a key hot path -- which it is, every modern network or disk I/O runs through these paths -- then it deserves at least _some_ consideration before adding more CPU cycles.
> On the other hand if the problem of passing wrong parameters here > isn't common I would be ok with dropping them.
As the author noted, it was only used in early platform bring-up. And simply reviewing the patch... it is clear that screwing up the parameters would cause massive, noticeable problems immediately -- such as on EM64T with swiotlb.
Jeff
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