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. -- 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
