I have hit the same problem on an hardy 64bit with a 3ware 9650SE raid controller
I found on 3ware site a (non optimal) workaround for my controller and a link for a patch on xen site (attached)

http://lists.xensource.com/archives/html/xen-changelog/2008-04/msg00008.html
http://www.3ware.com/kb/article.aspx?id=15345

Nov 12 14:41:14 fico kernel: [610002.809638] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.809715] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.809765] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.809821] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.809879] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.809937] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.809988] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.810037] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.810085] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.810134] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.810182] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.810231] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.810278] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.810327] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.810374] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.810434] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.810485] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.810534] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.810581] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.810630] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.810677] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.810725] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.810772] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.810820] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.810827] sd 0:0:0:0: [sda] Result: hostbyte=DID_ERROR driverbyte=DRIVER_OK,SUGGEST_OK
Nov 12 14:41:14 fico kernel: [610002.810830] end_request: I/O error, dev sda, sector 2616709092
Nov 12 14:41:14 fico kernel: [610002.810833] printk: 1098 messages suppressed.
Nov 12 14:41:14 fico kernel: [610002.810837] Buffer I/O error on device dm-11, logical block 1968989
Nov 12 14:41:14 fico kernel: [610002.810870] Buffer I/O error on device dm-11, logical block 1968990
Nov 12 14:41:14 fico kernel: [610002.810917] sd 0:0:0:0: [sda] Result: hostbyte=DID_ERROR driverbyte=DRIVER_OK,SUGGEST_OK
Nov 12 14:41:14 fico kernel: [610002.810919] end_request: I/O error, dev sda, sector 2616709220
Nov 12 14:41:14 fico kernel: [610002.825892] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0
Nov 12 14:41:14 fico kernel: [610002.825943] 3w-9xxx: scsi0: ERROR: (0x06:0x001C): Failed to map scatter gather list.
Nov 12 14:41:14 fico kernel: [610002.825996] PCI-DMA: Out of SW-IOMMU space for 61440 bytes at device 0000:0a:00.0