FUJITA Tomonori wrote: > On Thu, 17 Jul 2008 07:50:24 -0400 (EDT) > Mikulas Patocka <mpatocka@xxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
